Genotypic and phenotypic adaptation of pathogens: lesson from the genus Bordetella.

PURPOSE OF REVIEW: To relate genomic changes to phenotypic adaptation and evolution from environmental bacteria to obligate human pathogens, focusing on the examples within Bordetella species. RECENT FINDINGS: Recent studies showed that animal-pathogenic and human-pathogenic Bordetella species evolved from environmental ancestors in soil. The animal-pathogenic Bordetella bronchiseptica can hijack the life cycle of the soil-living amoeba Dictyostelium discoideum, surviving inside single-celled trophozoites, translocating to the fruiting bodies and disseminating along with amoeba spores. The association with amoeba may have been a 'training ground' for bacteria during the evolution to pathogens. Adaptation to an animal-associated life style was characterized by decreasing metabolic versatility and genome size and by acquisition of 'virulence factors' mediating the interaction with the new animal hosts. Subsequent emergence of human-specific pathogens, such as Bordetella pertussis from zoonoses of broader host range progenitors, was accompanied by a dramatic reduction in genome size, marked by the loss of hundreds of genes. SUMMARY: The evolution of Bordetella from environmental microbes to animal-adapted and obligate human pathogens was accompanied by significant genome reduction with large-scale gene loss during divergence.