Preservatives: Essential for Products, But Are They Disrupting Your Skin?

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.

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