One of the central goals of microbial forensics is to identify the source of a microorganism that has been used for terrorist or other illicit purposes. To achieve this goal, it is necessary to understand the extent of microbial diversity, the genetic processes that generate diversity, other evolutionary processes that shape the patterns of diversity, and the resulting genealogical relationships among diverse strains. The field of population genetics is concerned with precisely these issues and is therefore central to microbial forensics. 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.

An understanding of the population genetics of the human species already plays a major role in forensics. For example, by comparing the DNA "fingerprint" in a sample of tissue (e.g., blood) from a crime scene with the DNA from a suspect in that crime, one can say with a high degree of statistical certainty whether the suspect was the source of the forensic sample. Within an appropriate evidentiary context, a match in such a comparison provides powerful evidence of the suspect's involvement, whereas the absence of a match can exonerate an accused individual.

The power of DNA fingerprinting for forensics was not a forgone conclusion: instead there was intense debate for several years.' The eventual consensus that supported the utility of human DNA in forensics was reached after

Microbial Forensics

Copyright © 2005 by Elsevier Inc. All rights of reproduction in any form reserved.

systematic collection of data on the extent of diversity at the genetic loci used in testing, as well as detailed analyses of how that genetic diversity was distributed among different populations. This research has led to a greater basic understanding of the population genetics of the human species, as well as to improved forensic methods.

Given the successful forensic application of population genetic analyses of human data, it is not surprising that similar approaches are being pursued to trace the source of microorganisms (including bacteria, viruses, and fungi) whose genetic material is present in forensic samples. In fact, such work also represents a direct extension to forensics of the approaches that are widely used by molecular epidemiologists to track the source of outbreaks of many pathogens.

There are important differences in population genetics between humans and microorganisms. The aims of this chapter are to explain these differences, illustrate ways in which useful forensic inferences might be drawn from microbial DNA sequences, and suggest avenues for future research. Because microorganisms are themselves extraordinarily diverse, we will focus on bacteria and, as appropriate, use data on Bacillus anthracis to illustrate certain calculations. To start, we will compare and contrast inferences that can be drawn from DNA-based forensic evidence derived from humans versus bacteria.

0 0

Post a comment