Mirna Ghemrawi, Andrea Ramírez Torres, Michael Netherland Jr., Ying Wang, Nur A. Hasan, Bassam El-Fahmawi, George Duncan, Bruce McCord
Abstract
The microbiome is becoming an emerging field of interest within forensic science with high potential for individualization; however, little is known about bacterial species specific to the genital area or their ability to transfer between individuals during sexual contact. In this proof-of-concept study, we investigated microbial transfer dynamics in seven monogamous, heterosexual couples by collecting pre- and post-sexual intercourse samples from their genital areas, including penile, vaginal, and labial locations. Utilizing Shotgun Metagenomic Sequencing, we sequenced the microbial profiles of these samples. Our findings reveal significant transfer from the vaginal microbiome onto the penile microbiome, predominantly originating from the labial genitalia. Moreover, strain analysis unveiled distinct differentiation between the same species of bacteria across individuals, underscoring the potential for microbial forensics to distinguish individuals. This study contributes to our understanding of microbial transfer during sexual contact and highlights the forensic implications of the genital microbiome.
Introduction
This study addresses a common challenge in sexual assault casework, where sexual assault kits (SAKs) may fail to yield male DNA. Sexual assault is not always a sexually motivated crime, but rather one of power, control, and violence [1]. Because of this, often the suspect does not ejaculate, and when combined with factors such as the time difference between the attack and the collection of the SAK, there may be little if any foreign DNA to analyze [2]. For example, a report in San Diego, CA indicated that less than 40 % of sexual assault kits contained a foreign DNA profile that could be analyzed and submitted to CODIS. This would seem to imply that 60 % of victims are left without any scientific evidence to assist in their investigations, often leaving the assailant unprosecuted. However, we propose that the genital microbiome, and its potential for transferability, offer an alternative source of information to detect sexual contact.
The genital microbiome refers to the microorganisms that inhabit the genital region of males and females, including bacteria, fungi, and viruses, which are pivotal in maintaining genitourinary and reproductive health [3]. This complex ecosystem varies between individuals providing a potential source for discrimination [4], [5]. As a result, the human microbiome has been studied for applications in human identification and its potential transfer to various surfaces [6]. Despite its significance, there has been little research on forensic aspects of the genital microbiome, which physiologically differs from other body sites, and has potential value in cases of sexual assault. As per Locard’s Principle of Exchange, the genital microbiome may provide objective indication of sexual contact between two individuals, each of male and female sex, and then that can be used as evidence within criminal casework. A preliminary study conducted in our laboratory on penile and vaginal samples demonstrated that there was a significant difference between the penile and the vaginal microbiome profiles, where the former is significantly (at least 5 times more) diverse compared to the female vaginal cavity [7]. The acidity of the vaginal cavity (average pH = 4.0 +/- 0.5) is less when compared to other mammals which have pHs ranging from 5.4 to 7.8 [8]. This relatively low pH is due to the presence of lactobacilli which make up as much as 70 % of bacteria in the human vaginal tract while the level in other animals is around 1 % [8]. Undoubtedly this relatively low pH affects the selectivity of the vaginal cavity with respect to colonizing bacteria, protecting against vaginitis and certain types of sexually transmitted disease, however, it may also result from effects of human diet or other factors [9], [10].
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