Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2006
Publication Date: 4/20/2006
Citation: Bearson, S.M., Bearson, B.L., Rasmussen, M.A. 2006. Identification of Salmonella enterica serovar Typhimurium genes important for survival in the swine gastric environment. Applied and Environmental Microbiology. 72(4):2829-2836. Interpretive Summary: As a major food safety concern, Salmonella has been found in a variety of foods including dairy products, fruits, vegetables and meat products. According to the National Animal Health Monitoring System (NAHMS), over 34% of U.S. swine herds are infected with Salmonella. Since the harsh conditions of the stomach is one of the first environments that Salmonella will encounter in swine, this study used a genetic technique to identify Salmonella enterica serovar Typhimurium genes important for the survival of the organism in the presence of swine stomach contents. After exposing 1,598 signature-tagged mutants of Salmonella to an ex vivo swine stomach contents assay, 18 Salmonella mutants had a decrease in survival. As acidic pH is a major component of the stomach, each of the 18 identified mutants was analyzed for survival in a low pH study. Interestingly, only 3 of the mutants displayed a sensitivity to acidic conditions. Additional studies suggested that low pH is not the only lethal component in the swine stomach contents, with lactic acid a possible factor; however, the killing effect of the swine stomach contents was pH-dependent. These studies contribute to understanding the mechanisms that Salmonella uses to survive and colonize swine, potentially leading to improved diagnostic assays and control strategies.
Technical Abstract: As the stomach is the first line of defense for the host against ingested microorganisms, an ex vivo swine stomach contents (SSC) assay was developed to search for genes important for Salmonella enterica serovar Typhimurium (S. Typhimurium) survival in the hostile gastric environment. A mutation in phoP decreased the survival of S. Typhimurium by 10-fold following 20 min of challenge in the SSC assay, whereas no survival was observed for the rpoS or fur mutants at 20 min. A signature-tagged mutagenesis (STM) bank was constructed containing ~1600 mutants and screened in the SSC assay. Nineteen mutants (1.1%) were identified by differences in Southern hybridization patterns of the recovered STM pools to the input pools. Analysis of the identified mutants individually in the SSC and Acid Tolerance Response (ATR) assays revealed 13 mutants with a 10-fold or greater sensitivity in the SSC assay compared to the wild-type strain, but only 3 mutants with a 10-fold or greater decrease in survival following pH 3.0 acidic challenge. Examination of the SSC revealed that the lethal effects of the SSC are pH-dependent, but that low pH alone is not solely responsible for the killing mechanism(s). Gas chromatography analysis of the SSC revealed lactic acid levels of 151 mM. Upon investigating the effects of lactic acid on the survival of S. Typhimurium in a synthetic gastric fluid, a concentration and time-dependent lethal effect was observed. These studies indicate a role in gastric survival for several genes of S. Typhimurium and that the stomach environment is defined by more than low pH.