Microbiome-based body fluid identification of samples exposed to indoor conditions

In the forensic reconstruction of crime scene activities, the identification of biological traces and their bodily origin are valuable evidence that can be presented in court. While several presumptive and confirmatory tests are currently available, the limitations in specificity and sensitivity have instigated a search for alternative methods. Bacterial markers have been proposed as a novel approach for forensic body fluid/tissue identification. Bacteria are not only ubiquitous throughout the human body, but also, as shown by recent microbiome sequencing studies of the 16S rRNA gene, bacterial community structures are distinct across body sites. Traces and stains at crime scenes are, however, often exposed to the environment outside the human body for variable periods of time before laboratory processing. Thus, it is not clear whether exposed samples continue to harbor microbial signatures characteristic of their body site of origin. In this proof-of-concept study we collected samples from six different body sites: saliva, skin, peripheral blood, vaginal fluid, menstrual blood and semen. We exposed a subset of these samples to indoor conditions for 30 days while the remaining samples were processed directly after extraction. Our analyses of 16S rRNA gene sequence data for a total of 46 control and exposed samples show that both types of samples group by body site, although a few outliers are observed. Based on our results, vaginal and menstrual samples share their microbial signatures, and cannot be distinguished using bacterial markers. Overall, our findings indicate that bacterial markers are a promising avenue for forensic body fluid/tissue identification.