Inducible prophages contribute to Salmonella virulence in mice

We show that Salmonella typhimurium harbours two fully functional prophages , Gifsy-1 and Gifsy-2, that can be induced by standard treatments or, more effectively, by exposing bacteria to hydrogen peroxide. Curing bacteria for the Gifsy-2 prophage significantly reduces Salmonella's ability to establi sh a systemic infection in mice. Cured strains recover their virulence prop erties upon relysogenization. Phage Gifsy-2 carries the sodC gene for a per iplasmic [Cu,Zn]-superoxide dismutase previously implicated in the bacteria l defences against killing by macrophages. The contribution of the Gifsy-1 prophage to virulence - undetectable in the presence of Gifsy-2 as prophage - becomes significant in cells that lack Gifsy-2 but carry the sodC gene i ntegrated in the chromosome. This confirms the involvement of Gifsy-2-encod ed SodC protein in Salmonella pathogenicity and suggests that the Gifsy-1 p rophage carries one or more additional virulence genes that have a function al equivalent on the Gifsy-2 genome.