Challenges with co-amplification of microbial DNA in interpretation of STR profiles obtained from human skeletal remains.

STR-based DNA analysis is still the main tool for human DNA identification in most forensic DNA laboratories. DNA typing of aged human skeletal elements faces well-known interpretation challenges characteristic of degraded and low copy number DNA samples. Analyzing tens of thousands of human bone and teeth samples, we found that the occasional presence of artefactual peaks of presumed microbial origin adds another layer of complexity to the interpretation of STR profiles. In this paper, we present our approach and suggest guidelines for identifying and distinguishing non-human peaks, developed over the last 18 years. Additionally, we report a compendium of artefact peaks of presumed microbial origin recorded in human STR profiles obtained from bone and teeth samples, originating from Iraq, Chile, Maldives, Brazil and Western Balkans. Our experience has shown that these artefacts are not uncommon in bone STR testing, suggesting the possibility of occurrence in other forensic contexts, particularly trace DNA samples. Raising awareness among the forensic DNA community and accounting for this phenomenon is important for accurate STR interpretation.