SalmonellaHost Cell Invasion Emerged by Acquisition of a Mosaic of Separate Genetic Elements, IncludingSalmonellaPathogenicity Island 1 (SPI1), SPI5, andsopE2

Abstract

Salmonellaspp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of theSalmonellachromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverseSalmonellaspp., we show here that effector protein genes located in the core of SPI1 are present in allSalmonellalineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of theSalmonellachromosome, namely,sopB(SPI5, centisome 20),sopD(centisome 64), andsopE2(centisomes 40 to 42). Our data demonstrate thatsopB,sopD,andsopE2, along with SPI1, were already present in the last common ancestor of all contemporarySalmonellaspp. Analysis ofSalmonellamutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case ofSalmonella entericaserovar Typhimurium SL1344, by SopE: asopB sopEsopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], andsopE2) and that the last common ancestor of all contemporarySalmonellaspp. was probably already invasive.