After sexual intercourse, both partners leave traces of their own unique genital microbiome
Cell Press
Criminal investigations of heterosexual sexual assault often include a DNA analysis of the woman’s genitals with the aim of identifying the presence of the perpetrator’s sperm for proof of intercourse. However, in cases where no sperm is detected, including in assaults where the perpetrator uses a condom, these exams are often ineffective. In research publishing in the Cell Press journal iScience on February 12, 2025, researchers show that bacterial species are transferred between both individuals during sexual intercourse, and these species can be traced to a sexual partner’s unique genital microbiome. The authors say that analyses of these genital microorganisms—which they called the “sexome”—may be useful in identifying perpetrators of sexual assault.
“This research is based on the forensic concept that every contact leaves a trace,” says chief investigator Brendan Chapman of Murdoch University in Australia. “Until now, few studies have explored the vaginal and penile microbiomes within a forensic context. This research demonstrates that we can observe microbial traces from heterosexual couples’ genital microbiomes following sex.”
In this study, the researchers confirmed that both men and women have unique populations of bacteria in their genital areas. They then recruited 12 monogamous, heterosexual couples to investigate whether these sexomes are transferred during sexual intercourse, including when a condom is used. At the beginning of the study, each participant collected samples of their genital microbiome using swabs. The investigators used RNA gene sequencing to determine which bacteria strains were present—down to the sub-species level—and identified microbial signatures for each participant.
Couples were then asked to abstain from sex for varying lengths of time (from 2 to 14 days) and then to participate in intercourse. Afterwards, samples were collected again from each individual’s genital microbiome. Analysis showed that a participant’s unique bacterial signature could be identified in their sexual partner’s sample following intercourse.
Three of the couples reported using a condom. The analysis found that although this did have some impact on the transfer of microbial content, it did not inhibit it entirely. “When a condom was used, the majority of transfer occurred from the female to the male,” says Ruby Dixon of Murdoch University. “This shows promise for a means of testing a perpetrator post-assault and means there may be microbial markers that detect sexual contact even when a condom was used.”
The investigators also looked at whether males were circumcised and whether the participants had pubic hair, but found that neither factor seemed to affect the transfer of bacterial species between partners. However, they did find that the makeup of the vaginal microbiome changed during menstruation, which they note could affect results.
“The application of the sexome in sexual assault casework is still in its infancy,” Dixon says. “It’s important to completely understand the external factors that may have an impact on the microbial diversity of both males and females, and this is something we plan to continue studying.”
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This study was supported by the Murdoch University Bachelor of Science Honours program under the school of Medical, Molecular, and Forensic Sciences.
iScience, Dixon et al., “Bacterial transfer during sexual intercourse as a tool for forensic detection.” https://www.cell.com/iscience/fulltext/S2589-0042(25)00121-X
iScience (@iScience_CP) is an open access journal from Cell Press that provides a platform for original research and interdisciplinary thinking in the life, physical, and earth sciences. The primary criterion for publication in iScience is a significant contribution to a relevant field combined with robust results and underlying methodology. Visit: http://www.cell.com/iscience. To receive Cell Press media alerts, contact press@cell.com.
Journal
iScience
Method of Research
Experimental study
Subject of Research
People
Article Title
Bacterial transfer during sexual intercourse as a tool for forensic detection
Article Publication Date
12-Feb-2025
Study of female genital tract reveals key findings
Insights from an international team include identification of beneficial bacteria that thwart disease
Seeking to understand what constitutes a healthy vaginal microbiome, a global research collaboration that includes a Rutgers-New Brunswick scientist has reported a series of findings, including identifying which bacteria thwart vaginal disease and determining that microbiomes vary significantly across human populations.
Authors of the study, published in Trends in Microbiology, are part of a Belgium-based initiative called the Isala Sisterhood. Members of the group aim to inspire research on microbiomes worldwide by creating a “reference map” of vaginal microbiota. Launched in 2020 at the University of Antwerp, the project has expanded to include more than 3,000 participants working in North and South America, Asia, Africa and Europe. The group includes microbiologists, health care workers, members of governmental organizations and public citizens.
“The study is important for many reasons,” said Maria Gloria Dominguez- Bello, the Henry Rutgers Professor of Microbiome and Health at the Department of Biochemistry and Microbiology in the Rutgers School of Environmental and Biological Sciences and an author of the study. “One key reason is that understanding vaginal microbiome diversity can lead to improved diagnostics and treatments.”
Vaginas host a complex microcosm of bacteria and yeasts – known as the vaginal microbiome – that can fluctuate over time. However, little is known about these microbial communities and their roles in a person’s health. The project researchers are working to change that, with the study and its insights representing an important step forward, Dominguez-Bello said.
The bacteria Lactobacillus crispatus was identified by the researchers as a key protective bacterium in the vaginal microbiome. Not all strains, however, confer the same benefits, they found.
Aiming to explore the diversity of the vaginal microbiome in women across different geographical and ethnic backgrounds, the researchers confirmed that vaginal microbiomes vary significantly across populations. Lower levels of Lactobacillus species were more commonly reported in women of African and Latin American descent than in women of European and Asian descent.
They also sought to determine how variations in the vaginal microbiome correlated with women's health, particularly concerning bacterial vaginosis, the most common vaginal infection in women of childbearing age. Bacterial vaginosis, often defined by a reduction in Lactobacillus, isn’t necessarily a disease but a microbial imbalance that can have varying health consequences, the researchers said.
The study also touched on the issue of research gaps and potential biases. Dominguez-Bello said she and her colleagues are working to obtain data from more geographical areas as most studies focus predominantly on women from higher income countries. There is a need for broader global representation and more precise definitions of "healthy" vaginal microbiota, Dominguez-Bello said.
“The findings highlight disparities in women’s health research and advocate for a more inclusive approach that considers diverse populations,” she said. “By addressing gaps in research representation, the study encourages equitable distribution of scientific resources and expertise, especially in low- and middle-income countries.”
The authors emphasized the importance of continuing to study the vaginal microbiome. Largely unexplored, they said, are the roles of several other types of microbes that exist in vaginal microbiomes – including yeasts, viruses and other types of bacteria.
In addition, the microbiome’s connection to a person’s overall health is still not well understood and could be explored through studies that focus on geographical and socioeconomic diversity and consider social and cultural factors.
“To promote better preventive, diagnostic and therapeutic strategies for women affected by conditions associated with the vaginal microbiota, more research on the functions and diversity of the vaginal microbiota is urgently needed in different parts of the world,” the authors wrote. “This way, we can better understand what a healthy vaginal microbiome looks like in each geographical location.”
Journal
Trends in Microbiology
Method of Research
Survey
Subject of Research
People
Article Title
Diversity in women and their vaginal microbiota
Article Publication Date
11-Feb-2025
COI Statement
S.C.C. has received travel support from Copan to participate in two scientific conferences to present the Isala Sisterhood. S.A. is a voluntary academic board member of the International Scientific Association on Probiotics and Prebiotics, Students, and Fellows Association (https://www.isapp-sfa.com). S.L. and G.V. are academic board members of the International Scientific Association on Probiotics and Prebiotics (www.isappscience.org), but this organization was not involved in this article. M.C.C., S.L. and G.V. have received travel grants from ISAPP to participate in meetings. As a principal investigator (PI), S.L. has received research funding from several companies not involved in this article. She has been a scientific advisor for YUN and is now advising for Freya Biosciences. G.D. is the chairperson of Femicare vzw (https://www.facebook.com/profile.php?id=100063440664962) and has worked as a medical consultant for various industries. None of these organizations or companies was involved in the present article. M.T., J.K., K.C.A., V.P.C., M.G.D.B., C.M., S.M., and T.G. declare no competing interests.
