Individualization of pubic hair bacterial communities and the effects of storage time and temperature

A potential application of microbial genetics in forensic science is detection of transfer of the pubic hair microbiome between individuals during sexual intercourse using high-throughput sequencing. In addition to the primary need to show whether the pubic hair microbiome is individualizing, one aspect that must be addressed before using the microbiome in criminal casework involves the impact of storage on the microbiome of samples recovered for forensic testing. To test the effects of short-term storage, pubic hair samples were collected from volunteers and stored at room temperature (∼20 °C), refrigerated (4 °C), and frozen (-20 °C) for 1 week, 2 weeks, 4 weeks, and 6 weeks along with a baseline sample. Individual microbial profiles (R2 = 0.69) and gender (R2 = 0.17) were the greatest sources of variation between samples. Because of this variation, individual and gender could be predicted using Random Forests supervised classification in this sample set with an overall error rate of 2.7% ±  5.8% and 1.7% ± 5.2%, respectively. There was no statistically significant difference attributable to time of sampling or temperature of storage within individuals. Further work on larger sample sets will quantify the temporal consistency of individual profiles and define whether it is plausible to detect transfer between sexual partners. For short-term storage (≤6 weeks), recovery of the microbiome was not affected significantly by either storage time or temperature, suggesting that investigators and crime laboratories can use existing evidence storage methods.