Gender identity is not only a social experience: it also leaves a trace in biology. And not just in the brain or in hormones. A team at Emory University in Atlanta has found that the bacteria colonising the body’s mucous membranes differ according to a person’s gender identity, and that the pattern repeats itself on two different continents.
The study, published in Communications Medicine, brought together 205 HIV-negative participants in two far-apart cities. In Atlanta (United States) 58 people took part: 25 transgender women and 33 cisgender men who have sex with men. In Bangkok (Thailand), 147: 97 transgender women and 50 cisgender men from the same group. All of them provided rectal samples, and some Thai transgender women also provided neovaginal samples. Bacterial composition was analysed by sequencing the 16S ribosomal RNA gene.
The researchers also added a follow-up over time: 21 transgender women were sampled before and after starting feminising hormone therapy, to see whether gender-affirming hormones on their own changed the bacterial ecosystem.
The first result is clear: the rectal microbiota differs significantly both by gender identity and by geography. Transgender women showed an enrichment of bacteria capable of metabolising oestrogens in both cities, a pattern that holds on both sides of the Pacific. In cisgender men who have sex with men there was an enrichment of Prevotellaceae, but only in Atlanta: there, location weighed more than identity.
There is a nuance the authors emphasise. Overall bacterial diversity —how many different species are present— depended above all on where a person lives, not on gender identity. Put another way: geography sets how many bacteria there are; gender identity helps set which ones.
The neovaginal samples collected in Thailand painted a picture very different from the rectal one, dominated by bacteria associated with the skin and the gut and by anaerobic groups that earlier studies have linked to HIV seroconversion. A finding with direct clinical relevance for health care after vaginoplasty.
The second finding is more cautious: after short-term initiation of feminising hormone therapy, no significant changes were seen in the rectal microbiota. The authors attribute this to the possibility that, during that early period, hormone exposure may still be subtherapeutic. The longitudinal sample was small and the study is observational, so it cannot establish causality.
The conclusion they propose reaches beyond the microscope itself. According to the researchers, these data make it necessary to treat gender identity as a complex biosocial trait —biological and social at once— in HIV prevention, and they suggest that mucosal microbiota could influence transgender women’s vulnerability to infection.
It is the first time a study has compared identity, hormones and geography simultaneously in the microbiota of transgender people. And it leaves an underlying idea: when medicine measures, it is worth measuring who the person is as well.
Source: Biosocial influences of gender identity and geography on mucosal microbial phenotypes. Van Doren VE, Smith SA, Grimsley Ackerley C, et al. Communications Medicine 2026. DOI: 10.1038/s43856-026-01780-7.
Read the publication: Communications Medicine.
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