Bacteria evolve rapidly in response to the environment they encounter. Some environmental changes are experienced numerous times by bacteria from the same population, providing an opportunity to dissect the genetic basis of adaptive evolution. Here I discuss two examples in which the patterns of rapid change provide insight into medically important bacterial phenotypes, namely immune escape by Neisseria meningitidis and host specificity of Campylobacter jejuni. Genomic analysis of populations of bacteria from these species holds great promise but requires appropriate concepts and statistical tools.

DOI: 10.1371/journal.pgen.1000627
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