Isolation of a temperate bacteriophage encoding the type III effector protein SopE from an epidemic Salmonella typhimurium strain

Salmonella typhimurium employs the specialized type III secretion system en coded in pathogenicity island 1 (SP11) to translocate effector proteins int o host cells and to modulate host cell signal transduction. The SP11 type I II system and the effector proteins are conserved among all salmonellae and are thought to be acquired by horizontal gene transfer. The genetic mechan isms mediating this horizontal transfer are unknown. Here, we describe that SopE, a SP11-dependent translocated effector protein, is present in relati vely few S, typhimurium isolates. We have isolated a temperate phage that e ncodes SopE, Phage morphology and DNA hybridization, as well as partial seq uence information, suggest that this phage (SopE Phi) is a new member of th e P2 family of bacteriophages. By lysogenic conversion this phage can horiz ontally transfer genes between different S. typhimuriam strains. Strikingly , most of the isolates harboring SopE Phi belong to the small group of epid emic strains of S, typhimurium that have been responsible for a large perce ntage of human and animal salmonellosis and have persisted for a long perio d of time. Our data suggest that horizontal transfer of type III dependent effector proteins by lysogenic infection with bacteriophages (lysogenic con version) may provide an efficient mechanism for fine-tuning the interaction of Salmonella spp, with their hosts.