Can Bacteria Left Behind During Intercourse Serve as Forensic Evidence?

By Dr. Chinta SidharthanReviewed by Bethan DaviesFeb 21 2025

A recent study has explored the potential forensic applications of microbial signatures transferred between sexual partners. Researchers in Australia found that unique bacterial traces remain detectable after intercourse, even when condoms are used, highlighting a new avenue for forensic investigations when traditional DNA evidence is unavailable.

Study: Bacterial transfer during sexual intercourse as a tool for forensic detection. Image Credit: PeopleImages.com - Yuri A/Shutterstock.com

Microbes and Forensic Science

Microbiomes play a crucial role in human health and forensic science. Previous studies have demonstrated that each individual carries a unique microbial signature. Forensic investigations operate on the principle that every contact leaves a trace, suggesting that microbial transfer could be a valuable tool for criminal investigations.

Traditional forensic methods rely on human DNA to establish evidence of sexual contact. However, DNA degrades over time and may not always be detectable. While past research has identified microbial changes following intercourse, it has lacked the precision needed for forensic applications. Prior studies have also suggested that microbial signatures on the skin and hair can link individuals to crime scenes, but their use in sexual assault cases remains largely unexplored.

The Study’s Approach

To investigate bacterial transfer, researchers employed advanced sequencing techniques to track microbial changes post-intercourse. Their goal was to refine forensic methodologies, particularly for cases where traditional DNA evidence is insufficient.

The study involved 12 monogamous, sexually active couples between the ages of 20 and 30, ensuring all participants were free of sexually transmitted infections. Researchers collected vaginal and penile skin swabs before and after intercourse. To control for variations, participants abstained from sexual activity for 2 to 14 days before sample collection, and swabs were taken within 3 to 12 hours post-intercourse to reflect realistic forensic conditions.

Swab collection followed standardized procedures: female participants used sterile rayon swabs for low vaginal sampling, while males swabbed the penile shaft and glans using dampened swabs. Each couple contributed 20 swabs, five per time point.

Researchers then extracted DNA and sequenced the 16S ribosomal RNA gene, removing contaminant DNA before processing microbial sequences. They identified amplicon sequence variants (ASVs) and analyzed microbial diversity to assess bacterial richness and transfer. Statistical tests evaluated differences in bacterial diversity before and after intercourse, accounting for external factors such as condom use, oral intercourse, circumcision status, and pubic hair presence. Specialized software visualized microbial community changes between partners.

Key Findings

The study confirmed that sexual intercourse results in measurable bacterial transfer between partners. Unique microbial signatures from one partner were detectable in the other post-intercourse, providing strong evidence of bacterial exchange. The most frequently transferred bacteria were Lactobacillus species, a key component of the vaginal microbiome.

Interestingly, condom use did not entirely prevent bacterial transfer. While it reduced microbial exchange, ASVs still moved between partners, suggesting bacterial transfer occurs through mechanisms beyond direct skin contact. This finding has significant forensic implications, indicating that microbial evidence may persist even when conventional DNA markers are absent.

The study also found that circumcision status and pubic hair presence had no statistically significant effect on microbial diversity. Similarly, oral intercourse and lubricant use did not substantially alter microbial exchange patterns. These findings suggest that microbiome-based forensic evidence may be both stable and reliable.

However, individual variations in microbiome composition influenced bacterial transfer. Some couples showed greater microbial exchange than others, implying that factors such as immune response and hygiene habits may impact bacterial persistence. Despite these variations, microbial signatures remained distinct enough to differentiate individuals.

The findings suggest that microbiome analysis could become a valuable forensic tool, particularly in cases where DNA evidence is degraded or absent. The researchers emphasized the need for further studies to explore how long microbial signatures persist over time, which could help refine forensic applications.

Conclusion

This study provides compelling evidence that bacterial transfer during intercourse could serve as forensic evidence. The detection of unique microbial signatures post-intercourse supports the potential use of microbiome analysis in sexual assault investigations. While external factors had minimal impact on bacterial transfer, individual variations must be considered. Further research is needed to establish standardized forensic protocols and assess the long-term persistence of transferred microbial signatures.