Identification of Novel Salmonella Typhimurium Genes Required For Survival During Intestinal Inflammation

Bovine ligated ileal loops provide the best model to examine Salmonella Typhimurium genes required for survival during the early stages of infection, as humans and cattle develop very similar intestinal pathogenesis in response to the organism. Utilizing pools of mutants, both single gene and multi-gene deletion mutants, much of the S. Typhimurium genome can be screened simultaneously. A library consisting of multi-gene deletion mutants was screened in ligated ileal loops in calves. Regions of the genome required for survival in this model were identified in mucus and tissue samples using microarray analysis. STM1187-90 was one such region identified as under selection in both mucus and tissue. A ASTM1188 mutant was confirmed to poorly colonize the intestinal mucus and tissue in the presence of inflammation in competitive infections with the wild type. Complementation in trans reversed the phenotype of the ASTM1188 mutant. The phenotype of the ASTM1188 mutant was replicated and complemented in trans in the murine colitis model. STM1188 is a Salmonella specific gene whose protein product we show to be located in the inner membrane. This gene is absent in host-adapted serovars S. Typhi and S. Paratyphi A. Mutation of the putative lipobox cysteine to alanine resulted in mis- localization of STM1188C24A to the cytoplasm, and the inability to complement ASTM1188 in trans in mice. A second screen of a single gene deletion pool (SGD) identified many novel genes with potential roles during inflammation as well as many predicted genes with defined roles during the early stages of infection. Several novel gene phenotypes were confirmed and subsequently complemented in competitive infections with wild type. AhilE was identified during SGD screening and confirmed in bovine ligated ileal loops as being selected against during competitive infections with extensive inflammation, however, it was not selected against when the inflammatory immune response was limited. HilE has been previously shown to negatively regulate SPI-1 expression. Utilizing the murine colitis model, the AhilE phenotype was confirmed and complemented during competitive infection with wild type. By using (3-galactosidase assays, AhilE was confirmed to overexpress SPI-1, however, surprisingly AhilE also overexpressed SPI-2 during SPI-1 inducing conditions. In the current studies we performed Salmonella screens in bovine ligated ileal loops and confirmed novel virulence genes required for survival during inflammation.