Bacterial population genetics, evolution and epidemiology

Asexual bacterial populations inevitably consist of an assemblage of distin ct clonal lineages. However, bacterial populations are not entirely asexual since recombinational exchanges occur, mobilizing small genome segments am ong lineages and species. The relative contribution of recombination, as op posed to de novo mutation, in the generation of new bacterial genotypes var ies among bacterial populations and, as this contribution increases, the cl onality of a given population decreases. In consequence, a spectrum of poss ible population structures exists, with few bacterial species occupying the extremes of highly clonal and completely non-clonal, most containing both clonal and non-clonal elements. The analysis of collections of bacterial is olates, which accurately represent the natural population, by nucleotide se quence determination of multiple housekeeping loci provides data that can b e used both to investigate the population structure of bacterial pathogens and for the molecular characterization of bacterial isolates. Understanding the population structure of a given pathogen is important since it impacts on the questions that can be addressed by, and the methods and samples req uired for, effective molecular epidemiological studies.