Population Genetics of Bacteria in a Forensic Context

Population genetics can be viewed as a subfield of evolutionary biology that focuses on genetic diversity within species, as opposed to differences between species and higher taxonomic groups, although this distinction is not always precise, especially in microbes. It plays a major role in forensics of human species. This chapter explains differences in population genetics between humans and microorganisms, illustrating ways in which useful forensic inferences might be drawn from microbial DNA sequences, and suggests avenues for future research. In contrast to the human DNA-based forensics, a perfect match between a bacterial forensic sample and some potential source is less definitive, owing to exact clones that may exist. Such exact clones reflect the asexual mode of bacterial reproduction. Microbiology laboratories often store clones and exchange them with other laboratories. The opportunity for exact clones to confound forensic source tracking is thus especially relevant in the context of forensic samples that may have come from a laboratory. Even one or a few genetic differences between a forensic sample and some potential sources can exclude certain scenarios while focusing attention on others. The pattern of differences among potential sources might; for example, implicate one of them as an intermediate or final stage in the derivation of a forensic sample. The issues emphasized in this chapter are more relevant for tracking a deliberate attack than a natural epidemic. In the latter case, the number of generations and accumulated genetic differences are generally much greater, the potential sources are not deliberately stored as clones, and perfect attribution is often less critical.