Variable assortment of prophages provides a transferable repertoire of pathogenic determinants in Salmonella

Gene transfer between separate lineages of a bacterial pathogen can promote recombinational divergence and the emergence of new pathogenic variants. T emperate bacteriophages, by virtue of their ability to carry foreign DNA, a re potential key players in this process. Our previous work has shown that representative strains of Salmonella typhimurium (LT2, ATCC14028 and SL1344 ) are lysogenic for two temperate bacteriophages: Gifsy-1 and Gifsy-2. Seve ral lines of evidence suggested that both elements carry genes that contrib ute to Salmonella virulence. One such gene, on the Gifsy-2 prophage, codes for the [Cu, Zn] superoxide dismutase SodCI. Other putative pathogenicity d eterminants were uncovered more recently. These include genes for known or presumptive type III-translocated proteins and a locus, duplicated on both prophages, showing sequence similarity to a gene involved in Salmonella ent eropathogenesis (pipA). In addition to Gifsy-1 and Gifsy-2, each of the abo ve strains was found to harbour a specific set of prophages also carrying p utative pathogenicity determinants. A phage released from strain LT2 and id entified as phage Fels-1 carries the nanH gene and a novel sodC gene, which was named sodCIII. Strain ATCC14028 releases a lambdoid phage, named Gifsy -3, which contains the phoP/phoQ-activated pagJ gene and the gene for the s ecreted leucine-rich repeat protein SspH1. Finally, a phage specifically re leased from strain SL1344 was identified as SopE Phi. Most phage-associated loci transferred efficiently between Salmonella strains of the same or dif ferent serovars. Overall, these results suggest that lysogenic conversion i s a major mechanism driving the evolution of Salmonella bacteria.