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The skin microbiome is vital for skin health and barrier integrity. Sun exposure, especially UV radiation, plays a significant role in modulating this ecosystem. While moderate sun exposure aids vitamin D synthesis, excessive UV radiation disrupts microbial balance, causing oxidative stress and altering microbial composition. Understanding the interaction between UV and the skin microbiome is crucial for advancing skincare and overall skin health. What we know: A significant shift in microbial beta diversity was observed on the forearms of participants after four weeks of extensive sun exposure compared to baseline, suggesting that sunlight alters the diversity and composition of the skin microbiota (Willmott et al ., 2023). An overall increase in Cyanobacteria , Fusobacteria , Verrucomicrobia , and Oxalobacteraceae  species was observed, while Lactobacillaceae  and Pseudomonadaceae  species showed a decline after UVR exposure (Gilaberte et al ., 2025). Research shows that bacteria, like skin cells, react differently to UVA and UVB light. One study found both UV types reduce Pseudomonas aeruginosa, but Escherichia coli was less affected by UVA, indicating varying bacterial responses to sunlight (Smith et al., 2023). A study found that SPF 20 sunscreen protects both skin and its microbiome, preventing erythema and preserving beneficial bacteria like Lactobacillus crispatus. In contrast, unprotected or placebo-treated skin showed a disrupted microbial balance, with a reduced Lactobacillus to Cutibacterium acnes ratio (Schuetz et al., 2024). Applying sunscreen prior to UV exposure helps support and protect the skin microbiome, and researchers suggest that using sunscreens with higher SPF levels could provide even stronger microbial and skin protection (Schuetz et al ., 2024). Industry impact and potential: The growing awareness of how sun exposure affects the skin microbiome is driving innovation in sun care. Research indicates that UV protection can influence the balance of skin microorganisms, paving the way for products that not only shield against sun damage but also support overall skin health. Further research is needed to understand how different UV wavelengths impact the skin microbiome and contribute to long-term skin health issues, including aging and chronic conditions. More studies are also required to evaluate how various sunscreen formulations affect the skin’s microbial balance (Gilaberte et al ., 2025). Our solution: At Sequential, we help skincare brands create sun care products that protect the microbiome and support skin health. Through in vivo testing and detailed analysis of formulations' impact on the skin’s microbial ecosystem, we ensure products deliver UV protection without disrupting microbial balance. With access to over 20,000 microbiome samples, we provide scientifically-backed solutions that meet the growing demand for skin care prioritizing long-term health and immediate benefits. References: Gilaberte Y, Piquero-Casals J, Schalka S, Leone G, Brown A, Trullàs C, Jourdan E, Lim HW,  Krutmann J, Passeron T. Exploring the impact of solar radiation on skin microbiome to develop improved photoprotection strategies. Photochem Photobiol. 2025 Jan-Feb;101(1):38-52. doi: 10.1111/php.13962. Epub 2024 May 20. PMID: 38767119; PMCID: PMC11737011. Schuetz R, Claypool J, Sfriso R, Vollhardt JH. Sunscreens can preserve human skin  microbiome upon erythemal UV exposure. Int J Cosmet Sci. 2024 Feb;46(1):71-84. doi: 10.1111/ics.12910. Epub 2023 Oct 6. PMID: 37664974. Smith, M. L., O’Neill, C. A., Dickinson, M. R., Chavan, B., & McBain, A. J. (2023). Exploring  associations between skin, the dermal microbiome, and ultraviolet radiation: advancing possibilities for next-generation sunscreens. Frontiers in Microbiomes , 2 , Article 1102315. https://doi.org/10.3389/frmbi.2023.1102315 Willmott T, Campbell PM, Griffiths CEM, O'Connor C, Bell M, Watson REB, McBain AJ,  Langton AK. Behaviour and sun exposure in holidaymakers alters skin microbiota composition and diversity. Front Aging. 2023 Aug 8;4:1217635. doi: 10.3389/fragi.2023.1217635. PMID: 37614517; PMCID: PMC10442491.

The relationship between our oral and gut microbiomes is a growing area of research, offering new insights into how these communities shape health and disease. Emerging evidence is revealing how everyday oral hygiene practices, like antibacterial mouthwash use, affect this balance. What We Know: The gut and oral microbiomes are among the body’s largest microbial ecosystems, comprising 29% and 26% of the total bacterial count, respectively. Despite their distinct environments, their two-way connection - the ‘oral-gut microbiome axis’ - facilitates the exchange of microbial signals and metabolites that influence digestion, immune responses and systemic health. Disruptions in this axis have been linked to gastrointestinal disorders, cardiovascular diseases, among others, underscoring its vital role in maintaining overall health (Carvalho et al., 2024).  Although these microbiomes are distinct - due to barriers like gastric acidity and bile - oral bacteria may sometimes bypass these defences and migrate to the gut, influencing the gut microbiome and potentially contributing to diseases such as inflammatory bowel disease (IBD), colorectal cancer and systemic inflammatory conditions (Kunath et al., 2024). Industry Impact and Potential: Prolonged use of antibacterial mouthwash has been shown to disrupt the oral microbiome. A study on Listerine Cool Mint found that daily use for three months increased levels of Fusobacterium nucleatum  and Streptococcus anginosus . These opportunistic bacteria are linked to periodontal disease, systemic illnesses and even oesophageal and colorectal cancers. Moreover, oral bacteria that bypass the gut’s barriers may trigger systemic inflammation, compromising immune function and contributing to chronic diseases (Laumen et al., 2024). Research on chlorhexidine mouthwash in mice revealed notable changes in gut health, including reduced microbiome diversity, impaired nutrient absorption, and altered metabolism. While outcomes like decreased weight gain may initially appear beneficial, they are likely a result of malabsorption, which can have harmful downstream effects (Carvalho et al., 2024). These findings highlight the need to explore the oral–gut microbiome axis further, particularly the role of the oral microbiome in gut function and nutrient absorption. This opens new possibilities for developing oral hygiene products that maintain oral microbiome integrity while safeguarding the gut microbiome, paving the way for innovative solutions that support holistic health. Our Solution: At Sequential, we lead microbiome product development and testing from our hubs in London, New York and Singapore. We help businesses create products that preserve microbiome integrity while achieving efficacy. Partner with us to develop cutting-edge oral hygiene solutions that target the oral-gut microbiome axis and advancing health outcomes. References: Carvalho, L.R.R.A., Boeder, A.M., Shimari, M., Kleschyov, A.L., Esberg, A., Johansson, I., Weitzberg, E., Lundberg, J.O. & Carlstrom, M. (2024) Antibacterial mouthwash alters gut microbiome, reducing nutrient absorption and fat accumulation in Western diet-fed mice. Scientific Reports. 14 (1), 4025. doi:10.1038/s41598-024-54068-y. Kunath, B.J., De Rudder, C., Laczny, C.C., Letellier, E. & Wilmes, P. (2024) The oral–gut microbiome axis in health and disease. Nature Reviews Microbiology. 22 (12), 791–805. doi:10.1038/s41579-024-01075-5. Laumen, J.G.E., Van Dijck, C., Manoharan-Basil, S.S., de Block, T., Abdellati, S., Xavier, B.B., Malhotra-Kumar, S. & Kenyon, C. (2024) The effect of daily usage of Listerine Cool Mint mouthwash on the oropharyngeal microbiome: a substudy of the PReGo trial. Journal of Medical Microbiology. 73 (6). doi:10.1099/jmm.0.001830.

Folliculitis decalvans (FD) is a rare and challenging type of alopecia that leads to hair follicle inflammation, resulting in hair loss and scarring. Recent research suggests that FD has a unique microbiological signature and is associated with an impaired immune response, opening new avenues for understanding and treating this condition. What We Know: FD typically presents as a slowly expanding, painful alopecic plaque on the vertex of the scalp, often in young males. Despite extensive research, the exact cause is unclear. However, several factors have been implicated, including genetic predisposition, Staphylococcus aureus  colonisation, bacterial biofilms, compromised epidermal barrier integrity, congenital abnormalities in follicular orifices and dysfunction in the local immune system (Moreno-Arrones et al., 2023). As there is no definitive cure for FD, the goal of treatment is to stabilise the disease. Current therapeutic options include topical and systemic corticosteroids, antibiotics and isotretinoin. Case reports also highlight unconventional therapies such as topical tacrolimus, photodynamic therapy (PDT), dapsone, intravenous immunoglobulin (IVIG) and TNFα inhibitors, though these treatments are supported by limited evidence (Rózsa et al., 2024). Interestingly, while S. aureus  colonisation has long been linked to FD, recent research suggests its role may have been overstated due to past limitations in microbiological techniques. New studies reveal that FD-affected hair follicles have a distinct microbiome, with key species including Ruminococcaceae, Agathobacter  sp., Tyzzerella  sp. and Bacteroidales vadin  HA21 (Moreno-Arrones et al., 2023). Additionally, FD patients show significantly lower levels of IL-10, TNF-α and IL-6 after exposure to bacterial strains, indicating an impaired immune response that could contribute to the disease (Moreno-Arrones et al., 2023). Industry Impact and Potential: A successful case study treated a therapy-resistant FD patient with CO2 laser-assisted PDT. PDT induces fibroblast apoptosis, generates reactive oxygen species and offers antimicrobial and anti-inflammatory effects. Applying CO2 laser before PDT enhances photosensitiser absorption by creating microscopic channels in the skin. This method, previously effective for hypertrophic acne scars (Rózsa et al., 2024). Our Solution: With over 20,000 microbiome samples and 4,000 ingredients in our extensive database, along with a global network of more than 10,000 testing participants, Sequential offers comprehensive services to assess the impact of products and formulations. Our commitment to preserving microbiome integrity makes us an ideal partner for developing scalp and hair care products, including those focused on FD and scarring treatments. References: Moreno-Arrones, O.M., Garcia-Hoz, C., Del Campo, R., Roy, G., Saceda-Corralo, D., Jimenez-Cauhe, J., Ponce-Alonso, M., Serrano-Villar, S., Jaen, P., Paoli, J. & Vano-Galvan, S. (2023) Folliculitis Decalvans Has a Heterogeneous Microbiological Signature and Impaired Immunological Response. Dermatology (Basel, Switzerland). 239 (3), 454–461. doi:10.1159/000529301. Rózsa, P., Varga, E., Gyulai, R. & Kemény, L. (2024) Carbon-dioxide laser-associated PDT treatment of folliculitis decalvans. International Journal of Dermatology. 63 (9), 1256–1257. doi:10.1111/ijd.17136.

Menopause represents a significant hormonal shift, and its impact on the vaginal and vulvar microbiomes remains an area of emerging research. Given the prevalence of menopause-related conditions, understanding these changes is critical for advancing women's health and the treatment thereof. What We Know: Menopause introduces systemic symptoms and distinct changes in the vaginal microbiome, primarily driven by reduced estrogen levels. This reduction often leads to a decline in the dominant and favourable Lactobacillus  species, increasing the risk of microbial dysbiosis which is associated with further health complications including bacterial vaginosis, aerobic vaginitis, vulvovaginal candidiasis and increased risk of sexually transmitted infections (Muhleisen & Herbst-Kralovetz, 2016). Estrogen plays a vital role in regulating the vaginal microbiological environment by maintaining epithelial thickness and glycogen levels, promoting mucus secretion and lowering vaginal pH via Lactobacilli  colonisation and lactic acid production (Barrea et al., 2023) . These changes, along with shifts in the gut and oral microbiomes during menopause, are hypothesised to contribute to the development of menopause-related diseases, including osteoporosis, breast cancer, endometrial hyperplasia, periodontitis and cardiometabolic disorders. Therefore, interventions and solutions are crucial (Barrea et al., 2023) . Industry Impact and Potential: Hormone replacement therapy (HRT) has been shown to enhance Lactobacillus dominance in the vaginal microbiome, alleviating symptoms of dysbiosis. However, the negative side effects of HRT experienced by some patients mean that alternatives to this are necessary (Muhleisen & Herbst-Kralovetz, 2016) . Oral and vaginal probiotics hold great promise. Initial studies complement previous research findings on the menopause-vaginal microbiome connection, but additional trials are needed to determine the efficacy of bacterial therapeutics to modulate or restore vaginal homeostasis (Muhleisen & Herbst-Kralovetz, 2016) . In one study, a two-week oral supplementation with four Lactobacillus  species (two capsules daily) positively influenced vaginal microbiota colonisation in 22 postmenopausal patients undergoing chemotherapy for breast cancer. Although this is a small sample size, it highlights the potential of probiotic treatments (Marschalek et al., 2017) . Our Solution: In addition to vulvar microbiome analysis, we at Sequential provide services for assessing skin, scalp and oral microbiomes. We have established our company as a leader in facilitating the assessment and development of products that maintain microbiome integrity. Our team of experts is well-equipped to support your company in formulating innovative products and studies aimed at maintaining and improving the vulvar microbiome to support women’s health. References: Barrea, L., Verde, L., Auriemma, R.S., Vetrani, C., Cataldi, M., Frias-Toral, E., Pugliese, G., Camajani, E., Savastano, S., Colao, A. & Muscogiuri, G. (2023) Probiotics and Prebiotics: Any Role in Menopause-Related Diseases? Current Nutrition Reports. 12 (1), 83–97. doi:10.1007/s13668-023-00462-3. Marschalek, J., Farr, A., Marschalek, M.-L., Domig, K.J., Kneifel, W., Singer, C.F., Kiss, H. & Petricevic, L. (2017) Influence of Orally Administered Probiotic Lactobacillus Strains on Vaginal Microbiota in Women with Breast Cancer during Chemotherapy: A Randomized Placebo-Controlled Double-Blinded Pilot Study. Breast Care (Basel, Switzerland). 12 (5), 335–339. doi:10.1159/000478994. Muhleisen, A.L. & Herbst-Kralovetz, M.M. (2016) Menopause and the vaginal microbiome. Maturitas. 91, 42–50. doi:10.1016/j.maturitas.2016.05.015.

The vaginal microbiome undergoes profound changes during pregnancy, marked by shifts in microbial composition and diversity that significantly impact maternal health. While the importance of these shifts is increasingly recognised, the tools to interpret these changes remain limited. What We Know: The vaginal microbiome plays a pivotal role in pregnancy, with a healthy state predominantly featuring Lactobacillus  species. These bacteria help maintain a low pH, protecting against infections. Microbial dysbiosis is linked to complications such as preterm birth (PTB), miscarriage, gestational diabetes mellitus (GDM), preeclampsia and chorioamnionitis (CAT) (Gerede et al., 2024) . PTB is associated with increased levels of anaerobic bacteria like Gardnerella vaginalis and Prevotella . Communities dominated by L. iners  or anaerobic bacteria carry higher risks compared to  L. crispatus -dominant profiles. Similarly, miscarriage often correlates with reduced Lactobacillus  abundance and greater microbial diversity. Dysbiosis not only disrupts the protective functions of the microbiome but also promotes inflammation and tissue damage, which can contribute to complications such as cervical insufficiency or placental ischemia (Gerede et al., 2024) . In GDM, altered microbiota may exacerbate inflammatory pathways, worsening glucose intolerance. Elevated levels of Prevotella bivia  have been implicated in inflammation associated with preeclampsia, while a diverse microbiome depleted of L. crispatus  is linked to increased infection risks in CAT. These microbial shifts reflect dynamic interactions with maternal physiology and evolve across pregnancy trimesters (Parraga-Leo et al., 2024) . Industry Impact and Potential: Probiotic interventions to restore Lactobacillus  dominance show promise for managing bacterial vaginosis, but their efficacy in preventing broader pregnancy complications warrants further investigation. New evidence suggests that microbial profiles and community disruptions could serve as biomarkers for identifying high-risk pregnancies (Parraga-Leo et al., 2024). Recent innovations include the Vaginal Microbiome Atlas during Pregnancy (VMAP), which integrates data from 11 studies and 3880 samples across 1402 individuals. This comprehensive resource leverages MaLiAmPi, a cutting-edge phylogenetic tool implemented via a Nextflow pipeline, to harmonise diverse datasets. By addressing technical variations and improving accuracy, MaLiAmPi enhances the reliability of microbiome data, setting a new standard for microbiome analysis (Parraga-Leo et al., 2024). Our Solution: Sequential specialises in microbiome analysis, offering services for assessing the vulvar microbiome alongside skin, scalp and oral microbiomes. Our expertise in developing products that maintain microbiome integrity positions us as industry leaders in supporting innovations for women’s health.  References: Gerede, A., Nikolettos, K., Vavoulidis, E., Margioula-Siarkou, C., Petousis, S., Giourga, M., Fotinopoulos, P., Salagianni, M., Stavros, S., Dinas, K., Nikolettos, N. & Domali, E. (2024) Vaginal Microbiome and Pregnancy Complications: A Review. Journal of Clinical Medicine. 13 (13), 3875. doi:10.3390/jcm13133875. Parraga-Leo, A., Oskotsky, T.T., Oskotsky, B., Wibrand, C., Roldan, A., et al. (2024) VMAP: Vaginal Microbiome Atlas during Pregnancy. JAMIA open. 7 (3), ooae099. doi:10.1093/jamiaopen/ooae099.

Introduction The human microbiome is a significant driver of human health and disease, composed of trillions of microorganisms that contribute to supporting host health and development. While various factors play a role in influencing the overall diversity and composition of these communities, little remains known regarding the driving factors determining their inheritance and establishment (Benga et al. , 2024). Two main competing hypotheses exist to explain this: either the human microbiome is actively shaped by (1) host genetics, or (2) maternal transmission. Many studies seeking to resolve them have achieved mixed results, making it difficult to conclude on which is the primary driver of microbial inheritance. Regardless, recent findings on the skin and gut microbiomes now suggest that host genotype might play a more important role in the active shaping of certain microbial communities than initially thought (Benga et al. , 2024). Importance of the skin and gut microbiomes As the largest organ of the human body, the skin employs a variety of chemical, physical, and biological defences to protect the body from external stress or damage by acting as a barrier to infection, promoting thermoregulation, and preventing water loss (Smythe and Wilkinson, 2023). As an extra layer of protection, it has also evolved a specialised community of symbiotic microorganisms to carry out additional functions pertaining to human skin health known as the skin microbiome. With a density of 104  to 106 bacteria per square centimetre of skin surface (Cundell, 2018), it plays an essential role in promoting skin health by preventing growth of pathogens, priming the immune system to differentiate harmful microbes from friendly ones (Lunjani et al. , 2021), and even regulating skin growth and development (Meisel et al. , 2018). The gut is another key organ that possesses its own highly diverse and interconnected community of microorganisms, reaching densities as high as 1012 cells per gram depending on segment (Sekirov et al. , 2010). These microbes line the inner walls of the gastrointestinal tract like the stomach, small, and large intestine, where they aid in carrying out essential functions involving development of the human nervous (Dash, Syed and Khan, 2022) and immune systems, influencing host metabolic activity, fermenting food, and defending against pathogens (Hou et al. , 2022).  Influence of host genotype Several factors influence human microbiome composition over the course of an individual’s life. In most cases, these forces can act to introduce new species, increase or decrease their abundance, or completely wipe them out, which can affect host health in either a positive or negative direction. While we have a fairly comprehensive understanding of the environmental and endogenous factors modulating gut (e.g., immune system, diet) and skin (e.g., cosmetics, hormones) microbiome composition, less is known regarding the key factors influencing the active shaping of these communities in early life. So far two possible hypotheses have been proposed: (1) host genetics actively shape the microbiome, or (2) microbial inheritance occurs through maternal transmission.  Evidence that all humans (to date) share over 50 bacterial species across their gut microbiota despite other compositional differences is taken as evidence that there exists a core human gut metagenome responsible for preserving these groups across the human population (Boccuto et al. , 2023). Host-genetics are theorised to influence establishment of the gut microbiome through specific genes. Although the mechanisms of how it does so is not so well understood, some evidence points to these genes influencing certain physiological factors in the host body that affect gut landscape and resulting growth of microbes. For example, one study reported a strong association between the lactase gene and levels of Bifidobacterium , a bacterium that has evolved to digest sugars found in human and cow milk, with lactose-intolerant individuals possessing a higher abundance of this bacteria than lactose-persisters (Qin et al. , 2022). This is thought to be because their inability to metabolise lactose makes this sugar more easily available for consumption by bacteria in the gut compared to persisters that can break it down on their own, thus increasing their population (Goodrich et al. , 2016). As with the gut, microbial communities on the skin are thought to be influenced at some level by host genetic factors, albeit if similarly (if not more) understudied, with studies pointing to the influence of these genes on skin architecture (Si et al. , 2015) and its associated immune system (Srinivas et al. , 2013) affecting the ability of certain species to colonise the skin surface. For example, one study found a significant link between genetic variants related to deficient skin barrier function and an abundance of Corynebacterium jeikeium , a skin bacterium responsible for causing infection in immunocompromised patients, suggesting an impaired skin barrier results in poorer defence against pathogenic bacteria like C. jeikeium  that permit it to invade and cause disease more easily (Si et al. , 2015). Influence of maternal transmission Other sources point to the early establishment of an individual’s gut microbiota being primarily driven by maternal inheritance, with mode of delivery (vaginal or caesarean) being the main mechanism through which this occurs, however, the extent of its influence over the gut microbiome remains controversial. Some studies state vaginally-delivered infants possess more species characteristic of the mother’s vaginal ( Lactobacillus + Bacteroides ) and fecal microbiota ( Bifidobacterium ), while those delivered via C-section have a greater abundance of skin microbes such as Staphylococcus  (Wang et al. , 2024). However, these findings are inconsistent across studies, and some even suggest these effects are short-lived, with compositional differences between the two groups dropping to <2% within 5 years of an infant’s life (Bogaert et al. , 2023). Other proposed means by which maternal legacy shapes the gut microbiome is through breastfeeding, which transfers essential nutrients and beneficial microbes from the mother’s milk microbiome to the infant gut (Tian et al. , 2023), or placental transmission (Miko et al. , 2022) of microbes and microbial metabolites from the mother’s gut to the infant’s to seed the gut and prime the fetus’s primitive immune system to distinguish between friendly and harmful microbe strains. Similarly, mode of delivery has also been found to play a role in influencing the establishment of bacterial and fungal communities present on the infant skin, with one study reporting vaginally-born children possess more vagina-associated fungal groups ( Candida  and Rhodotorula ) than caesarean-delivered children that possess more skin-associated and airborne fungal genera ( Malassezia  and Alternaria ) (Wang et al. , 2022). Other studies have also reported differences in the bacterial composition of vaginally and caesarean-delivered children, with the former possessing more vaginal bacteria like Lactobacillus , and latter a greater abundance of skin bacteria, like  Staphylococcus, Corynebacterium , and Cutibacterium , indicating some level of influence in delivery mode in influencing skin microbiome abundance for the first 10 years of life (Dominguez-Bello et al. , 2010). Other studies however, have noted that microbial richness, diversity, or taxonomic profiles do not significantly differ between the cutaneous microbiomes of the two infant groups in the four weeks after birth (Pammi et al. , 2017), or even between vaginally and caesarean-delivered infants aged 1–3 months (Capone et al. , 2011). These observations are believed to be attributed to the highly dynamic nature of the newborn skin microbiome that resolves these differences over time, and also highlight the inconclusive nature of this data. Study: The host genotype actively shapes its microbiome across generations in laboratory mice (Benga et al.,  2024) Existing literature regarding the effects of maternal legacy (i.e., passage through the birth canal, weaning, coprophagy, and grooming) and host genotype on human microbiomes remain inconclusive. This study set out to determine which of the two factors plays a more important role in actively shaping host microbiome composition over several generations within a controlled setting, being one of the first studies to both look at host genotype effects on skin-based communities, and maternal effects across multiple generations, providing greater insight into its longer-term influences (Benga et al. , 2024).  Results The team collected early-stage embryos from two different mice strains, and by carefully controlling the environment to minimise its effect on the mice, bred them for six generations. The first generation of offspring were exposed to a common initial microbiome to observe how the effects of host genetics and maternal legacy would go on to alter composition over the next five generations, and disentangle these factors (Benga et al. , 2024). Figure 1: Schematic representation of microbiome inheritance across six generations of mice. (1) The study investigates whether microbiome composition is primarily shaped by maternal transmission or host genetics. (2) In early generations, maternal legacy plays a dominant role in microbiome composition. (3) Over successive generations, the influence of maternal transmission diminishes, and host genotype becomes the primary factor shaping microbiome structure, as indicated by the balance shifting from maternal legacy (blue) to host genotype (yellow) in later generations. Image taken from   (Benga et al., 2024). As illustrated in Figure 1, maternal legacy had a strong effect in shaping microbiome composition within the first generation of offspring, particularly for gut-based communities. However, its influence over both skin and gut microbiome composition weakened over time and was gradually overpowered by host genotype across subsequent generations. By F3 to F5, genetic factors became the dominant force in determining microbiome structure, as represented by the shifting balance in the schematic diagram. The study identified 33 microbial species that preferentially colonized hosts of specific genetic backgrounds, indicating genotype-specific enrichment of particular taxa. Furthermore, quantification of blood serum metabolites revealed significant differences in microbial metabolite abundance between host genotypes, suggesting an interaction between host genetics and microbiome function (Benga et al. , 2024). Conclusion The study suggests that under controlled environments, host genetic traits far outweigh any maternal impact on the gut microbiome, with genotype driving the active shaping of the host microbiome over several generations under controlled environmental factors. These effects could also possibly extend to the metabolic activity of the microbiome being modulated by host genetic factors, thus further shaping its behaviour and function. The study resolves the debate by showing that maternal legacy does not persist beyond the initial offspring generation in stable environments. This study by Benga et al. (2024) stands out as one of the few researches to explore both the effects of host genotype and the maternal influences on the microbiome. However, it is essential to acknowledge that findings from mice models may not fully translate to human physiology. Therefore, future studies should aim to replicate this research in human populations to better understand how host genotype and maternal effects interact to shape the microbiome.  Strengths and limitations Strengths: Improving our understanding of the factors modulating the composition and behaviour of the human microbiome has important implications for the identification of host disease markers and abnormal species growth that can interfere with microbiome function to cause disease, allowing the development of measures that mitigate against these host genotype-driven effects (Benga et al. , 2024) Understanding the factors influencing infant microbiomes and their role in the subsequent development of early immunity can catalyse the development of novel prebiotic/probiotic therapies that prevent pathogen colonisation and infection in vulnerable infant populations (Pammi et al. , 2017) Developing multi omic platforms that can analyse the metagenomic composition of individuals in relation to other components such as proteomics and metabolomics can help identify any genetic markers that could be associated with a dysbiotic microbiota and offer personalised solutions to help counteract and balance these effects  Limitations: Many of the studies looking into disentangling the effects of maternal and host genetic influence on microbiome composition remain inconclusive regarding the effects of either, with many studies concluding on the influence of host genotype being performed on immune defective or highly inbred mice, or lacking natural process of microbiome colonisation, instead relying on artificial methods not representative of actual microbial exposure in human infants (Benga et al. , 2024) These studies also fail to consider sites other than the gut microbiome, leaving a scarcity of information regarding the influence of maternal and host-specific factors on other communities in the body such as the skin, thus preventing any meaningful conclusions being drawn from gut-specific studies More longitudinal studies are needed to establish a stronger long-term link between these factors and their influence on human microbiomes, with most host genotype studies using murine models that may not accurately reflect human physiology, behaviours, and life history processes/child-rearing practices Implications & Applications Development of therapies to maintain the health of the microbiome in susceptible populations or reverse the dysbiotic effect of faulty genetics Knowledge of how maternal influence can affect microbiome and resulting infant health can empower caregivers to practice microbiome-friendly child rearing where possible, or encourage the development of similarly beneficial alternatives if not Combining genetics and microbiome screening approaches can allow for more accurate models to be drawn to predict individual therapeutic drug responses when treating dysbiotic microbiomes (Sanna et al. , 2022) Related Research and Future Directions Application of experiments seeking to establish a more causal relationship between host genotype and microbiome composition via studies implementing controlled interventions (e.g., genetic knock-outs, germ-free hosts) to better understand the genetic mechanisms controlling microbiome composition (Bubier, Chesler and Weinstock, 2021) Expanding upon the respective roles of maternal legacy and host genotype in influencing microbiome composition and shaping at other body sites such as the vaginal and oral microbiomes to understand how these can influence overall health and disease progression Extend this to see how host genotype can influence the relationship between microbiome dysbiosis and psychological health by studying the relationship between host genetics, microbiome composition, and any psychiatric disorder-associated phenotypes or endophenotypes Conclusion The skin and gut both harbour trillions of microbes that play a crucial role in the maintenance of health and regular bodily function, with numerous factors contributing to their composition. Host genotype is likely to prevail over the effects of maternal legacy when determining the initial formation and establishment of these microbial communities in early life, with maternal legacy effects only persisting in a single generation, after which they are overpowered and persisted by the host’s own genetic factors. Expanding these studies to other bodily sites, and more longitudinal ones, can help elucidate the extent to which these factors persist in their influence, as well as how they interact with or drive disease phenotypes (Benga et al. , 2024). At Sequential, we are at the forefront of microbiome research, revolutionizing the field through its innovative Multi-Omic Studies, which integrate human and microbiome analysis to uncover deeper insights into biological interactions. By employing state-of-the-art technologies, including genetic and metabolic profiling alongside advanced microbial sequencing, we provide a comprehensive understanding of how host genetics and the microbiome shape health outcomes. This multi-layered approach enables the development of science-baked formulations, enhances product efficacy, and advances personalized skincare solutions. With an extensive microbiome database and expertise in clinical testing, we are driving scientific progress in human-microbiome research. References Benga, L. et al.  (2024) ‘The host genotype actively shapes its microbiome across generations in laboratory mice’, Microbiome , 12(1), p. 256. Available at:  https://doi.org/10.1186/s40168-024-01954-2 . Boccuto, L. et al.  (2023) ‘Human Genes Involved in the Interaction between Host and Gut Microbiome: Regulation and Pathogenic Mechanisms’, Genes , 14(4), p. 857. Available at:  https://doi.org/10.3390/genes14040857 . 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(2022) ‘Understanding the Role of the Gut Microbiome in Brain Development and Its Association With Neurodevelopmental Psychiatric Disorders’, Frontiers in Cell and Developmental Biology , 10. Available at:  https://doi.org/10.3389/fcell.2022.880544 . Dominguez-Bello, M.G. et al.  (2010) ‘Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns’, Proceedings of the National Academy of Sciences , 107(26), pp. 11971–11975. Available at:  https://doi.org/10.1073/pnas.1002601107 . Ferretti, P., Pasolli, E., Tett, A., Asnicar, F., Gorfer, V., Fedi, S., Armanini, F., Truong, D. T.,  Manara, S., Zolfo, M., Beghini, F., Bertorelli, R., De Sanctis, V., Bariletti, I., Canto, R.,  Clementi, R., Cologna, M., Crifò, T., Cusumano, G., . . . Segata, N. (2018). Mother-to-Infant  Microbial Transmission from Different Body Sites Shapes the Developing Infant Gut  Microbiome. 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Eczema, or atopic dermatitis (AD), is a chronic inflammatory skin condition marked by skin barrier dysfunction and immune dysregulation. Driven by genetic, immunological and environmental factors, as well as skin microbiome changes, emerging research suggests live biotherapeutic products (LBPs) could revolutionise its treatment and prevention. What We Know: During AD flare-ups, microbial diversity declines and Staphylococcus aureus  often dominates. Up to 70% of individuals with AD have S. aureus  colonisation on lesional skin and 30%–40% on non-lesional skin. A disrupted skin barrier, due to genetic and environmental factors, increases pH and water loss, creating conditions for S. aureus overgrowth (Totté et al., 2016). The severity of AD symptoms correlates with S. aureus  levels, exacerbated by toxins like δ-toxin and PSMα. Notably, many AD patients’ microbiomes lack gram-negative bacteria, further reducing microbial diversity (Locker et al., 2024). LBPs, defined as live organisms used to prevent, treat, or cure diseases, offer a novel approach to addressing these imbalances (Ağagündüz et al., 2022). Industry Impact and Potential: LBPs show promise by targeting S. aureus  overgrowth and improving skin health. Examples include Roseomonas mucosa  and coagulase-negative staphylococci (e.g., Staphylococcus hominis  A9), which reduce S. aureus  through antimicrobial and immune-modulating mechanisms. Furthermore, Nitrosomonas eutropha  B244 produces anti-inflammatory nitrite, showing potential to alleviate AD  symptoms(Locker et al., 2024). @Concerto Bioscience recently initiated a Phase 1 trial of Ensemble No.2 (ENS-002), a topical LBP targeting S. aureus overgrowth. ENS-002 employs three microbial strains to address the root microbial deficiencies linked to AD. Designed for topical application, it minimises systemic risks like immune suppression or infections (Andrus, 2024). ENS-002’s development leveraged Concerto's kChip screening technology, which tests millions of microbial combinations to uncover interactions that modulate skin health. Using kChip, over 6 million microbial communities were screened to identify the "ensemble" of bacteria that neutralises pathogenic S. aureus. Our Solution: At Sequential, we specialise in Microbiome Product Testing to support your business’ goals, such as innovative AD treatments. Our tailored studies and product formulation support ensure developments that maintain microbiome integrity, promoting efficacy, compatibility, and healthier skin. Partner with us to confidently develop microbiome-based topical solutions that address AD’s unique challenges. References: Ağagündüz, D., Gençer Bingöl, F., Çelik, E., Cemali, Ö., Özenir, Ç., Özoğul, F. & Capasso, R. (2022) Recent developments in the probiotics as live biotherapeutic products (LBPs) as modulators of gut brain axis related neurological conditions. Journal of Translational Medicine. 20 (1), 460. doi:10.1186/s12967-022-03609-y. Andrus, E. (2024) Concerto Biosciences Announces First Participant Dosed with Live Biotherapeutic ENS-002 in Phase 1 Trial for Atopic Dermatitis. 2024. Concerto Biosciences. https://www.concertobio.com/press/concerto-biosciences-announces-first-participant-dosed-with-live-biotherapeutic-ens-002-in-phase-1-trial-for-atopic-dermatitis [Accessed: 19 November 2024]. Locker, J., Serrage, H.J., Ledder, R.G., Deshmukh, S., O’Neill, C.A. & McBain, A.J. (2024) Microbiological insights and dermatological applications of live biotherapeutic products. Journal of Applied Microbiology. 135 (8), lxae181. doi:10.1093/jambio/lxae181. Totté, J.E.E., van der Feltz, W.T., Hennekam, M., van Belkum, A., van Zuuren, E.J. & Pasmans, S.G.M.A. (2016) Prevalence and odds of Staphylococcus aureus carriage in atopic dermatitis: a systematic review and meta‐analysis. British Journal of Dermatology. 175 (4), 687–695. doi:10.1111/bjd.14566.

The ‘exposome’ refers to the complex interplay of environmental exposures that influence the skin over a lifetime, including factors such as pollution, UV radiation and lifestyle choices. Comparable in complexity to the skin microbiome, the exposome represents an exciting frontier in research, with significant implications for skincare innovation and personalised solutions. What We Know: The exposome encompasses a broad range of environmental and lifestyle factors: air pollution, UV radiation, climate, diet, sleep patterns, stress and hormonal changes. Each individual’s exposome is unique, shaped by the combination of these factors over time (Passeron et al., 2020). Key environmental elements such as traffic-related air pollution, hormones, nutrition, stress and sleep significantly impact skin ageing and overall skin health. For example, pollution accelerates pigmentation, wrinkles and eczema, while hormonal fluctuations, poor nutrition and stress contribute to inflammation, collagen degradation and conditions like atopic dermatitis, psoriasis and acne. These factors affect biochemical processes that influence skin ageing and the development of inflammatory skin disorders (Passeron et al., 2020). However, the skin’s exposome has been relatively underexplored and further investigation is needed to understand how these factors interact and the net effects they have on the skin (Krutmann et al., 2017). Industry Impact and Potential: As research into the exposome evolves, the skincare industry is increasingly focusing on how these factors drive skin ageing and health, leading to more personalised, targeted skincare solutions. One framework, called The Skin Interactome, integrates the genome, microbiome and exposome to unravel the molecular mechanisms underlying skin health and ageing (Khmaladze et al., 2020). This holistic approach examines how genetic, environmental and microbial factors work together to influence skin physiology. By identifying key molecular pathways, such as those involved in collagen synthesis, this framework aims to develop targeted strategies to protect skin health and delay the visible signs of ageing (Khmaladze et al., 2020). Pooling research across these distinct areas of skincare is vital, as it provides a comprehensive understanding of how environmental, lifestyle and biological factors collectively influence skin health and ageing. This integrated approach allows for the development of more targeted and effective skincare solutions. Our Solution: Sequential is at the forefront of microbiome research, supported by a database of 20,000 microbiome samples, 4,000 ingredients and a global network of 10,000 testing participants. Our customisable solutions span microbiome studies and product formulation, with a strong focus on preserving biome integrity. Whether exploring the skin, scalp, oral or vulvar microbiome, we are your ideal partner for advancing research. References: Khmaladze, I., Leonardi, M., Fabre, S., Messaraa, C. & Mavon, A. (2020) The Skin Interactome: A Holistic ‘Genome-Microbiome-Exposome’ Approach to Understand and Modulate Skin Health and Aging. Clinical, Cosmetic and Investigational Dermatology. 13, 1021–1040. doi:10.2147/CCID.S239367. Krutmann, J., Bouloc, A., Sore, G., Bernard, B.A. & Passeron, T. (2017) The skin aging exposome. Journal of Dermatological Science. 85 (3), 152–161. doi:10.1016/j.jdermsci.2016.09.015. Passeron, T., Krutmann, J., Andersen, M.L., Katta, R. & Zouboulis, C.C. (2020) Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology: JEADV. 34 Suppl 4, 4–25. doi:10.1111/jdv.16614.

Scalp psoriasis is a common yet often treatment-resistant autoimmune condition that frequently co-occurs with psoriasis in other areas. Currently, the specific influence of the scalp microbiome on scalp psoriasis, and how this can be leveraged for treatment, remains largely unexplored. What We Know: Psoriasis is a chronic inflammatory skin condition affecting 1–3% of the global population, characterised by persistent, scaly plaques. Genetic, environmental and epigenetic factors contribute to its development, with up to 80% of psoriasis patients experiencing scalp involvement (Choi et al., 2024).  Treatments for scalp psoriasis range from topical agents, including steroids and vitamin D analogues, to systemic treatments like methotrexate and cyclosporine. Despite available therapies, managing scalp psoriasis remains complex due to challenges with topical application and variability in patient response (Ghafoor et al., 2022) . Industry Impact and Potential: The skin microbiome of psoriasis patients differs significantly from that of healthy individuals. Psoriatic lesions exhibit increased Staphylococcus  and decreased Cutibacterium  compared to healthy controls. This dysbiosis may cause inflammation, impaired skin barrier functions and autoimmunity (Choi et al., 2024) . Researcher has shown that microbial diversity in the scalp microbiome increased with the severity of scalp psoriasis. Pseudomonas  and Malassezia  species, particularly M. globosa , were more prevalent in severe cases. Malassezia  is linked to several skin conditions, including psoriasis and its lipase activity may disrupt the skin barrier and provoke inflammation (Choi et al., 2024) .  Additionally, the IL-17 pathway, a key player in psoriasis pathogenesis, interacts with Malassezia  to exacerbate skin inflammation. Understanding these microbial changes offers a promising avenue for developing targeted treatments that address the root causes of scalp psoriasis, potentially enhancing patient outcomes (Choi et al., 2024) .  Powered by their Amino M³ Complex,™ @Act + Acre’s Microbiome Cooling Scalp Serum helps balance the scalp microbiome, soothing dryness, itching and reducing dandruff flakes. The formula uses peppermint oil for immediate relief, while amino acids, grape, ginger and frankincense restore microbiota balance and provide long-term protection against irritation. Our Solution: Sequential, with its database of over 20,000 microbiome samples and 4,000 ingredients, offers comprehensive services to evaluate product impacts on the microbiome. Our customizable microbiome studies, combined with real-world testing environments, provide critical insights into product efficacy. By partnering with Sequential, you gain access to data-driven solutions that help optimise your formulations and ensure they support scalp health in line with emerging research. References: Choi, J.-Y., Kim, H., Min, K.-H., Song, W.-H., Yu, D.-S., Lee, M. & Lee, Y.-B. (2024) Bacteria, Fungi, and Scalp Psoriasis: Understanding the Role of the Microbiome in Disease Severity. Journal of Clinical Medicine. 13 (16), 4846. doi:10.3390/jcm13164846. Ghafoor, R., Patil, A., Yamauchi, P., Weinberg, J., Kircik, L., Grabbe, S. & Goldust, M. (2022) Treatment of Scalp Psoriasis. Journal of drugs in dermatology: JDD. 21 (8), 833–837. doi:10.36849/JDD.6498.

Fasting, a practice rooted in history and religious traditions, has recently surged in popularity as a health trend. Its benefits - such as weight management, improved metabolic function and delayed ageing - are well-documented. However, new research suggests that fasting may also impact the oral microbiome, influencing oral health in unexpected ways. What We Know: Fasting involves abstaining from food, consuming only water or other approved liquids (e.g., herbal teas or black coffee) for an extended period of time. Different fasting types, such as intermittent fasting (less than 2 days) and long-term fasting (4 days to several weeks), have been studied clinically (Loumé et al., 2024). A lesser-known side effect of fasting is bad breath, or halitosis. This is often anecdotally linked to the "keto flu" during the body’s transition from burning carbohydrates to fat, but while ketone bodies may contribute to foul breath, this phenomenon differs from the pathological halitosis seen in some fasters (Loumé et al., 2024). Studies show that 80-90% of fasters with halitosis have oral microbiome dysbiosis. Oral microbes degrade residual proteins in saliva, food debris and shed epithelial cells, producing volatile sulphur compounds (VSCs) which are linked to halitosis, dysbiosis and periodontal disease (Loumé et al., 2024). Industry Impact and Potential: Recent research on long-term fasting’s effects on halitosis and the oral microbiome uncovered several key findings. Initially, fasting reduced microbial alpha diversity (a measure of species variety), but diversity rebounded and even exceeded baseline levels one and three months after fasting (Loumé et al., 2024). Fasting led to a decrease in genera such as Neisseria, Gemella  and Porphyromonas , while promoting an increase in others, including  Megasphaera, Dialister, Prevotella, Veillonella, Bifidobacteria, Leptotrichia, Selenomonas, Alloprevotella  and Atopobium . Firmicutes (Bacillota) became dominant during follow-up periods, while Proteobacteria  and Bacteroidetes  were suppressed (Loumé et al., 2024). The reduction in potentially harmful species like Porphyromonas  suggests a shift towards a less inflammatory microbial environment. Additionally, the correlation between microbial shifts and increased levels of dimethylsulfide - a compound linked to halitosis - indicates that fasting-induced changes in the microbiota may contribute to breath odour (Loumé et al., 2024). Our Solution: At Sequential, we are a trusted leader in microbiome product testing and formulation. Our customisable solutions empower businesses to innovate with confidence, ensuring the development of effective oral hygiene products that preserve the integrity of the oral microbiome. With our expertise, we help companies explore the potential of microbiome studies and product development not only for oral health but also for skin, scalp and vulvar microbiomes. References: Loumé, A., Grundler, F., Wilhelmi de Toledo, F., Giannopoulou, C. & Mesnage, R. (2024) Impact of Long-term Fasting on Breath Volatile Sulphur Compounds, Inflammatory Markers and Saliva Microbiota Composition. Oral Health & Preventive Dentistry. 22, 525–540. doi:10.3290/j.ohpd.b5795653.

Introduction: Cosmetic Preservatives Cosmetics are products designed to enhance or alter the appearance of the face, body, or hair, with effects such as hydration, anti-aging, whitening, and cleansing, depending on their intended purpose. Cosmetic formulations, consist of various ingredients that work together, including, base ingredients (water, surfactants, oils, polymers, emulsifiers) that provide texture and consistency, active ingredients such as hyaluronic acid for hydration or retinol for anti-aging, and colorants (natural or synthetic) that give pigmentation to the product (Tang & Du, 2024). Cosmetics are particularly water-based, and they can create a favorable environment for microbial growth. Therefore, “preservatives” are essential to prevent contamination of microbes and extend shelf life (Tang & Du, 2024). A well-designed preservation system, whether built into the formulation or added externally, should effectively prevent microbial contamination, maintaining product integrity from its sealed state until it is fully used, even after repeated exposure to air and contact (Halla et al ., 2018). Without preservatives, cosmetics could become breeding grounds for harmful bacteria and pose risks of infections or irritations, compromising both safety and effectiveness (Tang & Du, 2024). How do microorganisms contaminate cosmetics? Microbial contamination can occur at various stages, from production to consumer use, making it essential to identify and monitor all potential sources. Contamination during manufacturing (primary contamination) and during consumer handling (secondary contamination) can impact product safety and stability (Figure 1) (Halla et al ., 2018). To prevent microbial contamination in cosmetics, a comprehensive quality management system must be established, covering every stage from raw material selection to final consumer use. This includes strict quality control of raw materials, ensuring that only high-quality, contaminant-free ingredients enter the production process. Hygienic manufacturing processes, including proper cleaning, disinfection, and occupational hygiene, must be strictly followed to minimize the risk of primary contamination during production. Additionally, proper packaging and controlled distribution play a crucial role in preserving product integrity by preventing exposure to environmental contaminants (Uzdrowska & Górska-Ponikowska, 2023). Beyond manufacturing, consumer education on safe usage is equally important, as improper handling, such as using unclean applicators or exposing products to moisture, can introduce secondary contamination (Halla et al ., 2018).  Figure 1: Causes, effects, and prevention of cosmetic contamination. Image taken from   (Halla et al., 2018) . Microorganisms that have been identified in cosmetic products Cosmetic products can provide a favorable environment for microbial growth, particularly when produced under unsanitary conditions. Contamination often involves pathogenic bacteria such as Pseudomonas aeruginosa , Staphylococcus aureus , and Enterobacteria , which pose risks due to their ability to cause infections (Halla et al ., 2018).  Staphylococcus epidermidis  is another commonly detected microorganism linked to skin-related concerns (Alshehrei, 2023). Beyond bacterial contamination, fungal presence is a notable issue, especially Filamentous fungi , which are associated with opportunistic infections and mycotoxin production. Additionally, yeasts like Candida albicans  can proliferate in inadequately preserved formulations, increasing the likelihood of skin and mucosal infections (Halla et al ., 2018).  Although cases of infection from contaminated cosmetics are infrequent, studies have identified recurring microbial contaminants, including Klebsiella oxytoca , Burkholderia cepacia , Escherichia coli , Enterobacter gergoviae , and Serratia marcescens . While intact skin and mucous membranes serve as protective barriers, exposure to these pathogens may elevate infection risks, particularly for individuals with weakened immune systems or compromised skin integrity (Halla et al ., 2018). Regulations that have been placed for cosmetics As the cosmetic industry continues to evolve with the introduction of new ingredients, global regulations have been established to ensure consumer safety, product quality, and accountability for adverse reactions. Among the most influential regulatory frameworks are those from the United States, Japan, and the European Union, as these regions represent the largest cosmetic markets worldwide (Halla et al ., 2018).   United States Regulations In the United States, the Food and Drug Administration (FDA) oversees cosmetic safety and ensures that products entering the market comply with legal requirements. While the FDA does not mandate sterility in cosmetics, products must not contain pathogenic microorganisms, and the total microbial load must remain within safe limits. Although no strict microbial limits are specified by the FDA, the Personal Care Products Council (PCPC) provides industry guidelines, recommending that microbial contamination should not exceed 500 CFU/g for products intended for the eye area or infants and 1,000 CFU/g for all other cosmetics. Additionally, manufacturing facilities are expected to adhere to Good Manufacturing Practices (GMPs) to minimize the risk of microbial contamination (Table 1) (Halla et al ., 2018).       European Union Guidelines   In the European Union (EU), cosmetic safety is regulated under EU Council Directive 76/768/EEC. Microbiological safety guidelines are outlined by the Scientific Committee on Consumer Safety (SCCS), categorizing products into two groups:    Category 1: Products used on mucous membranes, eye area, or intended for children under three years old, where the microbial count must not exceed 100 CFU/g or CFU/mL (Halla et al ., 2018).       Category 2: All other cosmetic products, which must remain below 1,000 CFU/g or CFU/mL (Halla et al ., 2018).       Bacteria such as Pseudomonas aeruginosa , Staphylococcus aureus , and Candida albicans  are considered high-risk contaminants and must be absent in 1 g or 1 mL for Category 1 products and in 0.1 g or 0.1 mL for Category 2 products (Table 1) (Halla et al ., 2018).   Table 1: Regulations that have been placed for cosmetics. These regulations allow for strict quality control, hygiene practices, and microbial safety measures to be maintained in order to reduce contamination risks, ensuring the safety of cosmetic products for consumers. Preservation strategies in cosmetics   Cosmetic manufacturers use various strategies to prevent microbial contamination while maintaining product integrity. Preservation can be chemical, physical, or physicochemical, with two main stages:   1. Primary Preservation  - Implemented during manufacturing, focusing on Good Manufacturing Practices (GMP), including raw material quality control, water treatment, equipment sterilization, and hygienic environments (Halla et al ., 2018).   2. Secondary Preservation  - Applied after production to maintain stability during storage, transport, and consumer use. Different methods are used; Physical Preservation - Protective packaging, such as airless pumps, narrow openings, antimicrobial packaging, can minimise contamination (Halla et al ., 2018).     Physicochemical Preservation - Factors like water activity reduction (using salts, polyols, and hydrocolloids), emulsion type (W/O emulsions offer better protection than O/W), and pH control, that create an unfavorable environment for microbial growth (Halla et al ., 2018).     Chemical Preservation - Includes synthetic preservatives that are regulated under cosmetic laws, natural preservatives such as plant extracts and essential oils, and multifunctional ingredients such as, chelating agents, surfactants, and humectants that enhance microbial resistance while serving additional functions (Halla et al ., 2018).   The combination of these approaches, known as “Hurdle Technology”, ensures cosmetics remain safe, and free from pathogenic microorganisms (Halla et al ., 2018). The preservatives paradox Benefits of preservatives in cosmetics Preservatives play a crucial role in preventing the growth of bacteria, fungi, and other microorganisms, minimizing the risk of infections. Moreover, by maintaining product stability, preservatives help extend shelf life and ensure long-term safety for consumers. Preservatives also allow the development of diverse formulations, particularly those with high water content, which are more prone to microbial contamination (Tang & Du, 2024).  Challenges of preservatives in cosmetics Certain preservatives, such as parabens and isothiazolinones, have been associated with adverse health effects, including skin irritation, and to ensure effective preservation, certain formulas have high concentrations of preservatives, which further increase the risk of toxicity to consumers (Halla et al ., 2018).   Cosmetic preservatives are hazardous micropollutants that often remain in aquatic environments due to incomplete removal in wastewater treatment. These chemicals can harm organisms like fish and algae, raising concerns about their environmental impact (Nowak-Lange, Niedziałkowska & Lisowska 2022). Applying cosmetic products can disrupt the skin microbiota, affecting the balance of the skin, mucous membranes, and scalp. Products like moisturizers, soaps, shampoos, and lotions may alter the skin’s protective lipid layer and impact its natural microflora. This imbalance can result from factors such as preservatives, which remain active after application and interact with microbes, disturbing the bacterial balance (Pinto et al ., 2021). Preservatives impact on the skin microbiome Preservatives can impact the skin microbiome in several ways, and some of which are; Disruption of microbiota Certain preservatives can disrupt the microbiome balance of the skin. A study highlights that skincare products, particularly those with preservatives, influence the skin microbiome. The study found that preservative-free products (PFPs) led to an increase in commensal bacteria such as Sphingomonas   and   Neisseria , which are associated with UV protection, anti-aging effects, and reduced skin inflammation. In contrast, conventional skincare products (CSPs), which typically contain preservatives, showed different microbial shifts and were linked to a less pronounced positive impact on skin microbiome composition.(Wagner et al ., 2024). Disruption of microbial communication Some preservatives may interfere with quorum sensing, which is a way bacteria communicate using signaling molecules to coordinate group behaviors based on population density. Quorum sensing helps regulate functions like enzyme production, biofilm formation, and virulence (Falà et al ., 2022). By disrupting this process, preservatives can inhibit these functions and disrupt the microbial balance.  Alteration of skin pH Certain preservatives can alter the skin's pH, thereby influencing the growth of microorganisms. Maintaining the right pH is crucial for a healthy microbiome, as skincare products directly interact with both skin pH and microbial communities (Janssens-Böcker et al ., 2025).   Barrier function impact Some preservatives such as parabens and formaldehyde releasers, can weaken the skin barrier over time. Exposure to such ingredients may lead to barrier disruption, reducing the skin’s ability to retain moisture, defend against irritants, and maintain homeostasis, potentially causing sensitivity and irritation (Panwar & Rathore, 2024). Comprehensive review: Preservatives & the skin microbiome To gain a deeper understanding of the current knowledge on the impact of preservatives on the skin microbiome, two well-established publications have been selected. Study 1: Effect of commonly used cosmetic preservatives on skin resident microflora dynamics (Pinto et al ., 2021) The study investigated the effects of commonly used cosmetic preservatives (Table 2) on the skin microbiota using  in vitro methods. Methodology Table 2: List of formulations [C1-C12] containing preservatives used in this work. Table taken from (Pinto et al., 2021) . A solution was prepared by combining preservatives at standard concentrations with water and Aristoflex AVC (a gelling agent) to achieve a consistent viscosity. To mimic real skin conditions, a Labskin 3D skin model was utilized, enabling the researchers to observe interactions between the preservatives and the bacterial strains. The bacterial strains studied included Cutibacterium acnes , Staphylococcus epidermidis , and Staphylococcus aureus , which are key skin microbes. These strains were cultured under controlled conditions before being introduced to the 3D skin model.  Following bacterial inoculation, different preservative combinations were applied to the skin models to assess their impact on the skin microbiome. A preservative free gel was used as a control to determine baseline bacterial activity. After 3 hours of contact, two 4mm biopsy samples were collected from each skin model. RNA was extracted from the skin biopsies, and gene expression levels were analyzed using quantitative real-time PCR (qRT-PCR). Results  The results indicate that different preservative combinations have varying effects on skin microbiota (Table 3). Some combinations, such as C2 and C3, strongly inhibit S. aureus while moderately inhibiting C. acnes  without affecting S. epidermidis , making them ideal for restoring microbiome balance. Others, like C1, C4, C6, and C7, moderately inhibited S. aureus  and slightly inhibited C. acnes  while preserving S. epidermidis , suggesting their suitability for maintaining a balanced skin microbiome.   In contrast, combinations like C5, C8, and C9 significantly reduced S. epidermidis , which could disrupt the natural microbiota, making them less ideal for general cosmetic use. Notably, C10 strongly inhibited S. aureus  but had minimal impact on C. acnes  and S. epidermidis , making it a potential choice for targeting S. aureus driven dysbiosis. Table 3: Activity of the different combination of preservatives tested on growth dynamic expressed as % of inhibition. +++, strongly inhibited [< 75%]; ++, moderately inhibited [90–80%]; +, weakly inhibited [98–91%]; −, no inhibition. Table taken from (Pinto et al., 2021) . Conclusion In conclusion, the study shows that different preservative combinations have varying effects on the skin’s microbiome. Hence, choosing the right preservatives in cosmetics is important to keep the skin microbiome healthy and balanced.  There are several limitations of the study. The study only focuses on three bacterial strains Staphylococcus aureus , Staphylococcus epidermidis , and Cutibacterium acnes  which limits its ability to fully represent the diverse and complex skin microbiome. It does not account for the realistic effects of product usage or the dynamic interactions within the skin’s ecosystem. Additionally, the study does not reflect the full functionality of the skin, including its ability to regulate microbial balance. Importantly, it overlooks the microbiome’s natural resilience.  However, the findings provide valuable insights for formulators when having to choose preservatives for cosmetic products. Study 2: In-vivo  impact of common cosmetic preservative systems in full formulation on the skin microbiome (Murphy et al ., 2021) The study investigated the in vivo  impact of common cosmetic preservatives (Table 4) on the skin microbiome. Methodology Table 4: Ingredients of the 4 different formulations. In bold are the preservatives compositions. Table taken from (Murphy et al., 2021) . The study involved 60 healthy adult female participants aged between 18–55 from different ethnicities. 4 products were tested (3 rinse-off and 1 leave-on products), and each product was tested on a group of 15 participants, with no pre-conditioning phase to maintain their natural skin microbiome.  Leg skin microbiome samples were collected from each subject before and after application of the products, and the impact of the preservatives containing products was analyzed by using standard microbiome analysis including taxonomic and diversity analysis. Results: Alpha Diversity Alpha diversity analysis was performed to evaluate the preservatives impact on the  leg skin microbiome before and after product application, by using three common metrics, Chao1, Faith’s Phylogenetic Distance, and Shannon Entropy. All samples were standardized to a read count of 8000 based on rarefaction curve analysis.  Figure 2: Alpha diversity analysis of leg skin microbiome before and after product application. (A) Per sample alpha diversity assessment of the impact of the different preservation systems on the skin microbiome, (B) Group alpha diversity assessment of the impact of the different preservation systems on the skin microbiome, (C) Statistical analysis of alpha diversity changes between timepoints for each preservation system. Image taken from (Murphy et al., 2021) . As shown above (Figures, (A) Alpha diversity per sample for different preservation systems, (B) Group alpha diversity for preservation systems, (C) Statistical analysis of alpha diversity changes over time for each system), no significant changes in alpha diversity were observed after using the products. Results: Beta Diversity Beta diversity analysis was performed to evaluate the potential changes in the microbiome community structure across all product groups. The Bray-Curtis and Jaccard metrics were used to identify any significant shifts in the community after product use (Figures 3A–3D). No significant changes were observed. Fig 3. Beta diversity analysis of leg skin microbiome before and after formulation application. Analysis of the impact of cosmetic products containing different preservation systems (A-D) using Bray Curtis and Jaccard Diversity metrics. Panels A-D correspond to preservation systems A-D. Image taken from (Murphy et al., 2021) . Conclusion The study had several limitations. One being, the product's effectiveness diminishes due to dilution, particularly in wash-off products. Moreover, the study was limited to the leg skin microbiome, and does not apply to other body areas with different microbial compositions. Additionally, the study only focused on short-term effects, leaving the long-term impacts of continuous product use unclear. Finally, not including a preservative-free control group, limited the ability to fully assess the preservatives impact on the skin microbiome. However, despite these limitations the study was able to show us that the preservative containing products had no significant changes in the skin microbiome’s structure or diversity after usage. These data suggest that the different preservation systems in full formulation have minimal impact on the skin microbiome. The analysis also shows that the leg skin microbiome can recover to its original state after using the products. This was true for both wash-off products, which are diluted during use, and leave-on lotions, where the product stays on the skin longer without dilution.  In summary, preservatives are essential for ensuring cosmetic safety and stability, and both the in vitro and in vivo studies highlight the importance of selecting appropriate preservatives, and that when used in full formulation it has only minimal effects on the skin microbiome. However, more future research needs to be done to deepen our understanding of their influence on skin microbiome and health. References Alshehrei F. M. (2023). Isolation and Identification of Microorganisms associated with  high-quality and low-quality cosmetics from different brands in Mecca region -Saudi Arabia. Saudi journal of biological sciences , 30 (12), 103852. https://doi.org/10.1016/j.sjbs.2023.103852 Falà, A. K., Álvarez-Ordóñez, A., Filloux, A., Gahan, C. G. M., & Cotter, P. D. (2022).  Quorum sensing in human gut and food microbiomes: Significance and potential for therapeutic targeting. Frontiers in microbiology , 13 , 1002185. https://doi.org/10.3389/fmicb.2022.1002185 Halla, N., Fernandes, I. P., Heleno, S. A., Costa, P., Boucherit-Otmani, Z., Boucherit, K.,  Rodrigues, A. E., Ferreira, I. C. F. R., & Barreiro, M. F. (2018). Cosmetics Preservation: A Review on Present Strategies. Molecules (Basel, Switzerland) , 23 (7), 1571. https://doi.org/10.3390/molecules23071571 Janssens-Böcker, C., Doberenz, C., Monteiro, M., & de Oliveira Ferreira, M. (2025).  Influence of Cosmetic Skincare Products with pH < 5 on the Skin Microbiome: A Randomized Clinical Evaluation. Dermatology and therapy , 15 (1), 141–159. https://doi.org/10.1007/s13555-024-01321-x Murphy, B., Hoptroff, M., Arnold, D., Eccles, R., & Campbell-Lee, S. (2021). In-vivo impact of  common cosmetic preservative systems in full formulation on the skin microbiome. PloS one , 16 (7), e0254172. https://doi.org/10.1371/journal.pone.0254172 Nowak-Lange, M., Niedziałkowska, K., & Lisowska, K. (2022). Cosmetic Preservatives:  Hazardous Micropollutants in Need of Greater Attention? International Journal of Molecular Sciences , 23 (22), 14495. https://doi.org/10.3390/ijms232214495 Panwar, Aakash & Rathore, Priyanka. (2024). Impact of formulation excipients on skin  barrier functions: A review. International Journal of Pharmaceutical Chemistry and Analysis. 11. 41-44. 10.18231/j.ijpca.2024.005.  Pinto, D., Ciardiello, T., Franzoni, M., Pasini, F., Giuliani, G., & Rinaldi, F. (2021). Effect of  commonly used cosmetic preservatives on skin resident microflora dynamics. Scientific reports , 11 (1), 8695. https://doi.org/10.1038/s41598-021-88072-3 Tang, Zhenyu & Du, Qiaoyan. (2024). Mechanism of Action of Preservatives in Cosmetics.  Journal of Dermatologic Science and Cosmetic Technology. 1. 100054. 10.1016/j.jdsct.2024.100054.  Uzdrowska, Katarzyna & Górska-Ponikowska, Magdalena. (2023). Preservatives in  cosmetics technology. Aesthetic Cosmetology and Medicine. 12. 73-78. 10.52336/acm.2023.008.  Wagner, N., Valeriano, V. D., Diou-Hirtz, S., Björninen, E., Åkerström, U., Engstrand, L.,  Schuppe-Koistinen, I., & Gillbro, J. M. (2024). Microbial Dynamics: Assessing Skincare Regimens’ Impact on the Facial Skin Microbiome and Skin Health Parameters. Microorganisms , 12 (12), 2655. https://doi.org/10.3390/microorganisms12122655

Numerous deodorants and antiperspirant products have been developed to combat and treat malodour or body odour (BO). New approaches target the axillary (underarm) microbiome, the cause of BO, and meadowfoam extract could offer a natural solution. What We Know: Sweat is initially odourless and axillary malodor only develops when the cutaneous microbiome enzymatically breaks down sweat molecules produced by apocrine sweat glands. Certain molecules responsible for the odour associated with sweat have been identified, including an apical efflux pump encoded by the ABCC11  (MRP8) gene (Martin et al., 2010). Bacterium Staphylococcus hominis  has been identified as one bacterium that produces malodorous thiol compounds through the enzymatic degradation of sweat. The enzyme C-S lyase breaks down Cys-Gly-3M3SH, a peptide derivative found in sweat, into constituents including 3M3SH, which is a highly malodorous volatile compound from the thiol family. Due to its thiol nature, 3M3SH has a much lower olfactory threshold than other volatile compounds, making it the primary contributor to the intensity of perspiration odours (Verzeaux et al., 2024). White meadowfoam, Limnanthes alba , is a species of flowering plant native to California and Oregon that is known for its use in cosmetics and hair care products due to its stability, smooth texture and long-lasting presence on the skin (AgMRC, 2023).    Industry Impact and Potential: A deodorant containing meadowfoam extract was shown to be effective in reducing S. hominis. This product significantly reduced  S. hominis  abundance and C-S lyase activity, effectively decreasing odour without disrupting the axillary microbiome balance (Verzeaux et al., 2024). Traditional odour control methods - preventing sweat, masking odours with fragrance, or using antiseptic agents - face criticism due to physiological concerns, potential skin irritation, disruption of the axillary microbiota and the use of controversial ingredients like aluminium salts and alcohol. These issues highlight the need for risk-free, natural solutions that specifically target the biological mechanisms behind malodour production (Verzeaux et al., 2024).   Therefore, researchers propose meadowfoam-containing deodorants as a promising natural alternative for managing BO, with participants reporting high satisfaction in controlling both odour and perspiration (Verzeaux et al., 2024). Our Solution: Sequential is a leading expert in comprehensive, end-to-end microbiome product testing and formulation. Our specialised, customisable services enable businesses to develop innovative products that support and maintain microbiome health, ensuring both effectiveness and compatibility. We offer tailored expertise in facial, oral, scalp and vaginal microbiome research and formulation, providing full support for your product development needs, which may be extended to products targeting the axillary microbiome. References: Martin, A., Saathoff, M., Kuhn, F., Max, H., Terstegen, L. & Natsch, A. (2010) A Functional ABCC11 Allele Is Essential in the Biochemical Formation of Human Axillary Odor. Journal of Investigative Dermatology. 130 (2), 529–540. doi:10.1038/jid.2009.254. Verzeaux, L., Lopez-Ramirez, N., Grimaldi, C., Guedj, O., Aymard, E., Muchico, H. & Closs, B. (2024) Meadowfoam to Control S. Hominis and Axillary Malodor – As Shown by Meta Sequencing and Culturomics. Cosmetics & Toiletries. https://www.cosmeticsandtoiletries.com/cosmetic-ingredients/actives/article/22916729/silab-meadowfoam-to-control-s-hominis-and-axillary-malodor-as-shown-by-meta-sequencing-and-culturomics .