Genome Rearrangements inSalmonella

A small number of Salmonella enterica serovars are host specific, only causing infection in one host species or a few closely related species. The diseases caused by broad-host-range and host-restricted Salmonella serovars also differ. Rearrangements can occur via recombination between short repeat sequences, but most large chromosomal rearrangements in bacteria occur by recombination between repeated sequences with several kbp of homology. IS200 is the most common IS element in Salmonella, it transposes infrequently. Thus, in contrast to many bacteria that have a high background of transposition-mediated rearrangements, most genome rearrangements in Salmonella are due to recombination. In contrast to the broad-host-range serovars, isolates of host-specific S. entericaserovars have large-scale chromosomal rearrangements resulting from recombination between the rrn operons. Inversions and translocations change the order of the chromosomal regions between the rrn operons from the conserved arrangement type found in the broad-host-range serovars to one of over fifty arrangement types identified so far in host-specific serovars. Comparative genomics has revealed that many host-specific pathogens show greater genome plasticity than closely related bacteria that live in a wider variety of environmental conditions. Although the genome rearrangements in other bacteria are often mediated by active transposons, it seems likely that the observed differences in genome plasticity are simply determined by the selective constraints of their distinct ecological niches. Genome rearrangements also have practical implications for foodborne pathogens, as they can complicate the identification and tracking of outbreak strains.