• Transfer of unique and non-unique bacterial taxa during intercourse was observed
• Lactobacillus spp. contributed to the most female to male sexome transfer
• Despite the use of a condom during intercourse, transfer of the sexome was still observed
• Condom/lube use, oral intercourse, circumcision/pubic hair: no impact on microbial diversity

In forensic science, detecting transfers of physical and biological material is critical for establishing evidence of criminal involvement. Unique bacterial signatures from the reproductive system transfer during unprotected penetrative intercourse offer a novel tool for criminal investigation. Here, we demonstrate this transfer using full-length 16S rRNA gene sequencing and discuss the impact of barrier contraceptives. These microbial signatures can potentially aid in sexual assault casework for perpetrator identification when human male DNA is absent.