Evolution and Ecology of Salmonella.

Over the past 120 to 160 million years, the genus Salmonella has evolved into a complex group of more than 2,300 genetically and phenotypically diverse serovars. Members of this genus are able to infect a wide diversity of vertebrate and invertebrate hosts; disease manifestations in humans range from gastroenteritis to typhoid fever. The evolution of the genus Salmonella and the divergence and radiation of particular lineages within this group have resulted from selection acting on new genetic variation generated by events such as the gain, loss, and/or rearrangement of genetic material. These types of genetic events have contributed to the speciation of Salmonella from its ancestral association with cold-blood animals to a pathogen of warm-blooded hosts. Moreover, adaptive radiation due to changes in gene content within S. enterica subspecies I has impacted host specificity and aided in the selection of host-restricted, host-adapted, and non-host-adapted serovars. In addition to the genetic diversity important for the wide phenotypic heterogeneity within the genus, a subset of core Salmonella-specific genes present in all Salmonella species and serovars has been identified that may contribute to the conserved aspects of the lifestyle of this microorganism, including the ability to survive in nutrient-poor nonhost environments such as soil and water. Whole-genome comparisons of isolates differing in host range and virulence will continue to elucidate the genetic mechanisms that have contributed to the evolution and diverse ecology of the genus Salmonella.