Exploring Microbiome Shifts in Transgender Men: The Impact of Transition on Vaginal Health

Introduction

The vagina is one of the most heavily colonised organs of the human body, with a unique ecosystem consisting of bacteria, fungi, viruses and other groups of microorganisms that play a vital role in modulating reproductive fertility, preventing inflammatory diseases and sexually transmitted infections, and may even contain microbial biomarkers indicating risk of preterm delivery during pregnancy (Lee et al., 2023). Its physical properties (i.e., low pH and oxygen) make it an ideal environment for specific colonisation by groups of mostly acid-favouring and low-oxygen tolerant species, resulting in a relatively low level of diversity (France et al., 2022; Lee et al., 2023).  

In healthy, cisgender women (individuals whose gender identity is the same as their birth-assigned sex), the vaginal microbiomes is usually dominated by a single group of bacteria known as the Lactobacilli, which are capable of producing lactic acid compounds that work to maintain the acidic pH of the vagina and inhibit growth of harmful pathogens (Huang et al., 2024). This low diversity composition is favoured within the vagina, as shifts from a Lactobacillus dominant to a more diverse microbiome are commonly associated with increased risk of disease and infection, including disorders such as sexually transmitted infections (STIs), bacterial vaginosis (BV), and even HIV (France et al., 2022; Feil et al., 2024).

The vaginal microbiomes of transgender men

Transgender men (i.e., individuals assigned female sex at birth but identify as male) who have retained their natal genitalia may also choose to undergo gender affirming hormone therapy (GAHT) in the form of testosterone supplementation to aid in presenting with a more masculine appearance through increased facial and body hair, greater muscle mass, and suppression of menstruation (Winston McPherson et al., 2019). 

While much of the focus of vaginal microbiome research has been related to cisgender women, it is also worth focusing on the properties of these communities in transgender individuals, especially those that might be undergoing testosterone therapy, as this hormone is predicted to play a strong role in influencing the composition of the vaginal microbiome with substantial effects. However, the relationship between the two remains to be fully explored, with only a couple of studies in the current literature seeking to understand it.

Study No. 1: The vaginal microbiome of transgender men (Winston McPherson et al., 2019)

To better understand the effects of GAHT on vaginal microbiome composition, this study set out to investigate how testosterone would go on to influence the vaginal floras of a cohort of healthy transgender men prescribed testosterone for at least 1 year compared with samples taken from cisgender women, being one of the first studies in the field to do so (Winston McPherson et al., 2019).

Results

The researchers found the vaginal flora of most the transgender men to have a lower abundance of Lactobacillus (<2%) as the primary bacterial group inhabiting the vagina compared to the microbiomes of cisgender women (>90%), and a greater overall bacterial diversity and abundance of species such as Gardnerella and Prevotella associated with increased risk of bacterial vaginosis (BV). 

However, transgender individuals receiving oestrogen either as a treatment for vaginal atrophy or BV showed a positive association with the majority presence of Lactobacillus (>90%) and reduced species diversity in the microbiota, suggesting a similar effect of oestrogen in maintaining a favourable environment for Lactobacillus colonisation and prevention of disease in transgender men. The authors go on to state that administration of intravaginal oestrogen might counteract these effects and restore balance to the vaginal microbiomes of transgender men receiving testosterone therapy (Winston McPherson et al., 2019).

Conclusion

Testosterone can act to cause compositional changes in the vaginal microflora of transgender individuals by depleting Lactobacillus abundance and promoting the growth of bacterial species associated with bacterial vaginosis, leading to differences between cisgender women and transgender men. This study also draws a link between oestrogen and Lactobacillus, with the former promoting growth and colonisation of the vagina by the latter and reducing diversity, suggesting a possible therapy for the treatment of conditions associated with this kind of dysbiosis (Winston McPherson et al., 2019).

Study No. 2: The vaginal microbiome of transgender men receiving gender-affirming hormonal therapy in comparison to that of cisgender women (Feil et al., 2024)

Building off results from previous studies, the aim of this was to investigate similarities between the vaginal microbiome compositions of transgender men and menopausal and premenopausal cisgender women as as the effects of hormonal testosterone therapy in the former and reduced oestrogen in the latter are believed to have a similar effects in both groups (Feil et al., 2024).

Results

Analyses of microbiome composition revealed transgender men and menopausal women to possess greater species diversity than premenopausal women, with the vaginal communities of transgender men characterised by similarities to those of menopausal women, a reduction in Lactobacillus and increase in the population of gut-associated species such as Campylobacter, Anaerococcus, Dialister, and Prevotella. However, the abundance of the latter two groups showed a decline over the duration of hormonal therapy in trans men.

The authors of the study suggest these similarities between transgender men and menopausal women to be driven by a reduction of oestrogen in the blood resulting in a reduction of vaginal glycogen, a vital chemical metabolised by Lactobacillus species into lactic acid that maintains the ideal acidic environment of the vagina. As this oestrogen decreases, less glycogen is available as a food source for these beneficial bacteria, causing the population to decrease and the vaginal pH to rise as a result. This opens up room for colonisation by other species, thus causing the observed increase in species diversity, and increasing susceptibility to infection.

Over time, the authors noted a reduction in this species diversity with length of testosterone treatment in transgender men, likely caused by a lowered abundance of Dialister and Prevotella species, and suggesting a shift to a less diverse vaginal microbiome with prolonged testosterone therapy. Although they did note that Lactobacillus populations failed to return to their original dominance even after this period (Feil et al., 2024).

Conclusion

The study suggests that the reduced abundance of Lactobacillus and overall increase in species diversity within the vaginal microbiomes of transgender men receiving GAHT to be driven by a reduction in glycogen compounds in the vagina that Lactobacillus species use as a food source, with their subsequent loss opening up space for habitation by other species and infection, and resulting in effects similar to those in menopausal cisgender women while differing significantly from the microbiomes of premenopausal cisgender women (Feil et al., 2024).

Study No. 3: Characteristics of the Vaginal Microbiome Before and After Testosterone Treatment in Transgender Men (Panichaya et al., 2024)

Another study looking to investigate the effects of initiating testosterone therapy on the composition of vaginal microbiota in transgender men by comparing vaginal communities before and after testosterone use over the course of 12 weeks in a cohort of Thai participants, while also assessing its impact on the appearance of vulvovaginal symptoms such as vaginal pH, vaginal atrophy score (VAS), and vaginal maturation value (VMV) (Panichaya et al., 2024).

Results

This study also reported a loss of Lactobacillus dominance post-testosterone treatment, accompanied by a significant increase of Prevotella and Streptococcus. Similar to previous studies, administration of testosterone resulted in an increase in the vaginal microbiome diversity of transgender men in the post-treatment group, further lending support to the composition-altering effects of testosterone on these microbial communities. 

Participants of the study reported the appearance of more vulvovaginal symptoms after 12 weeks of testosterone treatment, with higher VAS, higher vaginal pH, and worse VMVs, however these symptoms did not demonstrate any statistically significant correlation with the decreased relative abundance of Lactobacilli observed in these groups, with the authors suggesting a potential trend that could be further elucidated through future studies with larger sample sizes. Interestingly, the study also failed to establish any significant statistical correlation between changes in hormone levels within the participants (i.e., decrease in estradiol/increase in testosterone) and reduction of Lactobacillus, another observation that merits being followed up on.

While there were no reports of infection in the 12-week follow up after the study had ended, the authors predicted longer term use of testosterone might eventually cause vaginal infection and other physical symptoms (e.g., painful intercourse, itching, irregular bleeding) to emerge (Panichaya et al., 2024).

Conclusion

This study demonstrated significant changes to occur in the vaginal microbiomes of transgender men undergoing testosterone therapy, including a reduction in the relative abundance of Lactobacillus and increase in overall diversity, two symptoms commonly associated with potential adverse vaginal health outcomes. It also looked at physical effects resulting from hormonal therapy in relation to these compositional changes in the vaginal microbiota, and reported an interesting trend emerging between the two despite their lack of significant correlation (Panichaya et al., 2024).

Strengths & Limitations of Research

Strengths:

Improving our understanding of how testosterone therapy can influence the vaginal microbiomes of transgender men can drive the development of strategies to prevent or reduce the risk of serious infection or disease such as BV/HIV commonly associated with testosterone-altered microbial communities. This will aid in improving the quality of life and healthcare outcomes for transgender individuals undergoing GAHT, and also improve sexual health within this population.

Developments in this field will also help destigmatize discussions and research surrounding vaginal health in transgender men so that individuals and healthcare professionals can accurately address concerns surrounding these topics, while also helping transgender individuals make more informed decisions regarding their health.

Data from these studies can provide extensive repositories of vaginal and serum specimens collected from transgender participants that can be used by researchers as a resource to accelerate progress in fields such as disease research, drug development, and biomarker identification within the context of transgender health research (Muzny et al., 2023).

Limitations:

The small sample sizes used in these studies reduces their ability to identify subtle differences between groups, draw clear correlations between hormone levels and vaginal microbiomes, while increasing the likelihood of obtaining statistical errors that could reduce the accuracy of conclusions being drawn from the data (Winston McPherson et al., 2019).

Many of the aforementioned studies also failed to collect any demographic information (race, ethnicity, or body mass index) on their participants, meaning little information could be obtained on the extent of these factors in influencing the rate or magnitude of testosterone-driven changes in vaginal microbiome composition (Winston McPherson et al., 2019).

More longitudinal studies looking into the effects of testosterone on the vaginal microflora of transgender men are needed. These will help better define the relationship between the two, and establish a stronger causal link between any observed compositional changes (Lactobacillus depletion; increase in diversity) and testosterone therapy, should one exist.

Related Research and Future Directions

The findings of these studies can be taken further through the development of therapeutic treatments to treat unwanted effects in the vaginal microbiomes of transgender individuals receiving GAHT, such as the use of vaginal or oral probiotics to prevent infection by restoring balance to the microbiome without the use of oestrogen therapy that can have potentially dysphoric effects (Feil et al., 2024).

Understanding the hormonal factors influencing the vaginal microbiome of transgender men may have potential therapeutic applications in developing approaches to restore microbiome balance in other groups. This may include those of menopausal women possessing similar compositions to transgender individuals, the neovaginal microbiomes of transgender women who have not yet started oestrogen therapy to establish these Lactobacillus dominant communities, as well as cisgender women suffering from hormonal disorders such as polycystic ovary syndrome (PCOS) resulting in above average levels of testosterone that might cause similar microbiome shifts as those observed in transgender men.

Gathering more demographic data (e.g., ethnicity/age/race) could help determine whether these factors affect how vaginal microbiomes respond to testosterone therapy and improve the generalisability of studies investigating testosterone’s influence on these microbial communities (Panichaya et al., 2024).

Conclusion

The vagina is an incredibly complex organ housing trillions of microorganisms that play an essential role in its healthy development. However, many studies looking into the role of the vaginal microbiome have almost exclusively focused on these effects in cisgender women, with scarce information on how their role could be affected during gender-affirming testosterone therapy in transgender men. Despite this, recent findings suggest testosterone to be a big player in altering the composition of the vaginal microbiome from its healthy state of Lactobacillus dominance to a more diverse one that runs the risk of causing infection or disease. More studies are needed to better understand this relationship, improve our knowledge of transgender health, and drive the development of effective treatments to minimise any risk of harm arising from these testosterone-mediated shifts in microbiome structure.

References

Feil, K. et al. (2024) ‘The vaginal microbiome of transgender men receiving gender-affirming hormonal therapy in comparison to that of cisgender women’, Scientific Reports, 14(1), p. 21526. Available at: https://doi.org/10.1038/s41598-024-72365-4.

France, M. et al. (2022) ‘Towards a deeper understanding of the vaginal microbiota’, Nature Microbiology, 7(3), pp. 367–378. Available at: https://doi.org/10.1038/s41564-022-01083-2.

Huang, L. et al. (2024) ‘A multi-kingdom collection of 33,804 reference genomes for the human vaginal microbiome’, Nature Microbiology, 9(8), pp. 2185–2200. Available at: https://doi.org/10.1038/s41564-024-01751-5.

Lee, C.Y. et al. (2023) ‘New perspectives into the vaginal microbiome with systems biology’, Trends in Microbiology, 31(4), pp. 356–368. Available at: https://doi.org/10.1016/j.tim.2022.09.011.

Muzny, C.A. et al. (2023) ‘Impact of testosterone use on the vaginal microbiota of transgender men, including susceptibility to bacterial vaginosis: study protocol for a prospective, observational study’. Available at: https://doi.org/10.1136/bmjopen-2023-073068.

Panichaya, P. et al. (2024) ‘Characteristics of the Vaginal Microbiome Before and After Testosterone Treatment in Transgender Men’, Transgender Health [Preprint]. Available at: https://doi.org/10.1089/trgh.2023.0249.

Winston McPherson, G. et al. (2019) ‘The Vaginal Microbiome of Transgender Men’, Clinical Chemistry, 65(1), pp. 199–207. Available at: https://doi.org/10.1373/clinchem.2018.293654.