Helicobacter pylori evolution and phenotypic diversification in a changing host

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Abstract
The human gastric pathogen Helicobacter pylori displays a high degree of intraspecies allelic diversity and variability. Almost every infected person carries one or multiple unique H. pylori strains that can be readily distinguished by MLST or other typing methods. Diversity within H. pylori is generated by the unusual combination of an elevated mutation rate and frequent interstrain recombination during mixed infections. Unusually short DNA fragments are incorporated into the H. pylori genome in the course of recombination events, further contributing to allelic diversification. Modern H. pylori bacteria can be subdivided into six main populations with distinct geographic distribution patterns. These modern populations are derived from five ancestral populations, and the distribution of ancestral nucleotides over the globe reflects ancient and more recent human migrations. Many H. pylori genes contain hypermutable sequences, such as homopolymeric nucleotide repeats. Any large H. pylori population will therefore consist of multiple subpopulations with specific activity patterns for these so-called contingency genes (bacterial quasispecies). Genetic variability that is due to intrastrain diversification and interstrain recombination is hypothesized to help the bacteria adapt to individual hosts after transmission. Although experimental evidence is still scarce, this concept is supported by the finding that extensive genetic and phenotypic variation is displayed by molecules involved in interactions with the human host, including adhesins, lipopolysaccharides and components of the cag type IV secretion apparatus, including the translocated effector CagA.