Today, on 25 November, the International Day for the Elimination of Violence Against Women, we want not only to talk about the magnitude of the problem, but also about what can be improved in practice. A few years ago, I told you about our GeneDoe project and the microbial fingerprint. Now, for the first time, we can look back with concrete numbers: what can semen, the vaginal microbiome, and even a pair of underwear reveal after sexual violence?
Last year, the UN campaign stated: “Every 10 minutes a woman is killed. #NoExcuse.”
Our response as a research team? Ensuring that forensic tools become better, gentler, and fairer. That means: believing survivors, informing them that clothing can also be used as evidence, and developing new techniques that bring stories of violence to light and strengthen their credibility.
From crime scene to citizen science
My journey began in the Forensic Biomedical Sciences program at KU Leuven, where every trace at a crime scene has the potential to become evidence. When I started my PhD with our very first Isala daughter project, GeneDoe, I asked myself a new question: Can women’s microbiomes (i.e., the microorganisms in the vagina and on the skin) tell us not only something about their health, but also about what happened to them if they experienced sexual violence?
Thanks to more than 3,000 Isala participants and the ten brave GeneDoe volunteers, we can now answer that question. Our full study has just been published in Microbiome, a leading scientific journal in microbiome research. In this blog, I’d like to share some highlights. You can read the full article here if you want to dive deeper, and here you’ll find a short video summarizing the findings.
What was GeneDoe again?
One in five women in Belgium reports having been raped. Many survivors do not file a report due to trauma or stigma. And if they do, it is often too late for classical DNA evidence, which may already have disappeared.
Our hypothesis? Microbial DNA remains detectable for longer and withstands time and temperature better than human DNA. Just as we leave behind a unique human DNA fingerprint, we also each have a unique microbial fingerprint — one that varies depending on the place on our body, lifestyle, environment, and recent activities like sex.
This idea of the microbiome as a forensic tool builds on our earlier review of microbial fingerprints in forensic science, in which we discussed the potential of vaginal, skin, and saliva microbiomes (see our review in Frontiers in Microbiology)
During GeneDoe, we conducted an intensive intervention study with ten women who, for a week, each day:
– Wore identical cotton underwear, which they exchanged daily and sent in for analysis
– Collected a vaginal sample each morning
– Had sex with their partner on specific days (day 1 and day 5)
Sounds intimate? It was. But it was crucial to understand how sex acts as a moment of exchange of semen, cells, and microbes between partners, and between body and underwear.
Thanks to these courageous women, we could examine not only the effect of sex on the microbiome, but also how long the microbial changes last, what happens after showering, and whether underwear captures the same traces as vaginal swabs.
Want to know more about the GeneDoe project? Read my earlier blogs here and here.
Microbiomes as an extra piece of the puzzle
A breakthrough in our research: with 71% accuracy, we can predict whether someone has had sexual intercourse within the last 24 hours based on the composition of the vaginal microbiome. Three specific bacteria turn out to be key: Streptococcus, Staphylococcus, and Peptoniphilus. They increase after sex and act like temporary “witnesses” of what happened. In the controlled GeneDoe study of ten women, accuracy was even higher at 82%.
But the real test came when we applied this model to 44 forensic samples from sexual violence cases — samples stored for years in the forensic DNA labs of Antwerp. The result: 77% accuracy.
This type of activity-level evidence, telling us what happened and when, could complement the classic question of “Whose DNA is this?” as long as it is carefully validated.
“For the first time, we could use the microbiome not only to ask ‘Whose is this?’ but also ‘What activity took place?’”
Underwear as silent witness
There was another surprising discovery — one we hinted at in an earlier blog, but can now confirm with solid data: underwear appears to be an even better biological witness than vaginal swabs.
In the GeneDoe study, underwear was at least as effective — and often better — than vaginal swabs in detecting semen with a PSA test (a prostate-specific antigen test that measures a protein made only by the prostate and found in semen):
- 80% of underwear tested positive the morning after sex (vs. 90% of vaginal samples)
- 70% still tested positive after showering and putting on clean underwear (vs. 50% of vaginal samples)
- Even after 72 hours at room temperature, all underwear samples still tested positive
This matters enormously. Why? Because 34.8% of survivors shower or bathe before seeking medical help, and vaginal sampling is often experienced as invasive during sexual assault examinations.
We also confirmed this pattern after analyzing 207 real cases of sexual violence: clothing and objects (condoms, bedsheets…) yielded 1.8 times more positive semen detections, and more usable male DNA profiles than samples taken directly from the body.
But, there’s a but… and that’s important:
It’s not perfect — and we must say that clearly. 23% of our microbiome predictions were false negatives, meaning sex did occur, but the model failed to detect it. Reasons may include time since the incident, individual variation, or other factors.
So, this is not a miracle solution. It is an additional tool.
A predictive model can never stand as the only piece of evidence. The fear that survivors will not be believed remains real. What we can provide is an extra puzzle piece that can be used alongside testimony, DNA analysis, and other evidence. And we ensure that new techniques like microbiome analysis are not used against survivors, but to support them.
Science in the service of justice
What makes this research so special? You. The more than 6,000 women who participated in Isala. Without your reference dataset, we could never have understood what “normal” looks like. Without knowledge of variation due to age, antibiotics, menstrual cycle… we would be nowhere. Citizen science is not just a nice term — it’s the foundation on which we scientists build. From your bathroom to the forensic lab. From taboo to tool.
Our study is now published in Microbiome. But publication is not the end. It is the starting point for further validation, implementation, and hopefully (one day) routine use in forensic laboratories. Meanwhile, we keep working, keep asking questions, keep breaking taboos. Because violence against women is not only a legal or medical issue. It is also a scientific problem — one we can help find solutions for. Every time a survivor is heard, every time a case is solved, every time evidence makes the difference between justice and impunity… it matters.
Want to know more?
- Our new study: Ahannach et al. (2025). “Microbial and seminal traces of sexual intercourse and forensic implications” in Microbiome
- Our earlier review on microbiome use in forensic science: Ahannach et al. (2021). “At the interface of life and death: post-mortem and other applications of vaginal, skin, and salivary microbiome analysis in forensics” in Frontiers in Microbiology
- Previous blogs: GeneDoe project and Underwear as information source.
Survivor of sexual violence? You are not alone.
Free, confidential help is available 24/7:
- 1712: call, chat, or email with a professional
- www.seksueelgeweld.be: information and help, also anonymously
- Care Centres after Sexual Violence: medical care, forensic examination, legal support
You do not have to go through this alone. There is help. There is support. And justice is possible.
Who am I?
I am Sarah Ahannach, Isala project manager and microbiologist at the University of Antwerp, with a passion for forensic research. With a background in forensic biomedical sciences and research into the female microbiome, I aim to build bridges between the lab and society so that scientific evidence no longer silences women but strengthens them.
