Phylogenetic analysis as a tool in molecular epidemiology of infectious diseases.

Phylogenetics is a powerful tool for microbial epidemiology, but it is a tool that is often misused and misinterpreted by the field. Microbial epidemiologists are cautioned that in order to draw any inferences about the order of descent from a common ancestor it is necessary to correctly root a phylogenetic tree. Epidemiological samples of microbial populations typically include both ancestors and their descendants. In order to illustrate the relationships of those isolates, the phylogenetic method used must be able to detect zero-length branches. Unweighted Pair-Group Method (UPGMA) is the phylogenetic method that is most widely used in microbial epidemiology. Because UPGMA cannot detect zero length branches, and because it places the root of the tree based on a usually-false assumption, UPGMA is the worst possible choice among the several phylogenetic methods available. Because microbial epidemiology deals with relationships among strains within a species, rather than with relationships among species, recombination within those species can render phylogenetic trees meaningless and positively misleading. When there is evidence of significant recombination within the species of interest phylogenetic trees should not be used at all. Instead, alternative tools such as eBURST should be used to understand relationships among isolates.