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Most crime scenes are not sterile and therefore may be contaminated with environmental DNA, especially if a decomposing body is found. Collecting biological evidence from this individual will yield DNA samples mixed with microbial DNA. This also becomes important if postmortem swabs are collected from sexually assaulted victims. Although genotyping kits undergo validation tests, including bacterial screens, they do not account for the diverse microbial load during decomposition. We investigated the effect of spiking human DNA samples with known concentrations of DNA from 17 microbe species associated with decomposition on DNA profiles produced using the Promega PowerPlex® HS system. Two species, Bacillus subtilis and Mycobacterium smegmatis, produced an extraneous allele at the TPOX locus. When repeated with the PowerPlex® Fusion kit, the extra allele no longer amplified with these two species. This experiment demonstrates that caution should be exhibited if microbial load is high and the PowerPlex® 16HS system is used. [ABSTRACT FROM AUTHOR]