Identification of a pathogenicity island required for Salmonella enteropathogenicity

Salmonella spp. interact with ileal mucosa and disrupt normal intestinal function, which results in an acute inflammatory cell influx, fluid secretion and enteritis. We have recently characterized SopB, a novel secreted effector protein of Salmonella dublin, and presented evidence that SopB is translocated into eukaryotic cells via a sip-dependent pathway to promote fluid secretion and inflammatory responses. Here, we show that sopB is located on a large DNA fragment unique to the Salmonella chromosome. This locus is conserved in Salmonella and maps at approximately 20 centisome of the S. typhimurium chromosome. Sequence analysis revealed that this Salmonella-specific DNA fragment is flanked by DNA sequences with significant sequence similarity to the Escherichia coli K-12 genes, tRNA1ser (serT) on one side and copS/copR on the other. Thus, this Salmonella-specific DNA fragment has features characteristic of 'pathogenicity islands' and, therefore, it was denoted SPI-5 (Salmonella pathogenicity island-5). SPI-5 was sequenced and was found to contain five novel genes, pipA, pipB, pipC, pipD (pathogenicity island-encoded proteins) and orf, in addition to sopB. The effect of mutations in pipA, pipB and pipD on the induction of fluid secretion and an acute inflammatory cell influx was assessed in bovine ligated ileal loops. The effect of mutations in SPI-5-encoded genes on systemic salmonellosis was assessed in mice. The results of these experiments suggest that SPI-5-encoded genes contribute to enteric but not to systemic salmonellosis.