Submitted to: American Society for Microbiology
Publication Type: Abstract Only
Publication Acceptance Date: 5/22/2006
Publication Date: 5/22/2006
Citation: Bearson, B.L., Bearson, S.M., Rasmussen, M.A. 2006. Sensitivity of Salmonella enterica serovar Typhimurium to lactic acid present in porcine stomach contents [abstract]. American Society for Microbiology. p.639.
Technical Abstract: As the stomach is a hostile environment and a first line of defense for the host against foodborne microorganisms, an ex vivo swine stomach contents assay was developed to search for genes important for Salmonella enterica serovar Typhimurium survival in the porcine gastric milieu. Challenge of previously identified, acid-sensitive S. Typhimurium mutants in the swine stomach contents assay revealed a 10-fold decrease in survival for a phoP mutant (macrophage survival and antimicrobial peptide resistance) following 20 minutes of challenge, whereas no survival was observed for mutants of rpoS (alternate sigma factor) or fur (iron metabolism regulator). The pH of the swine stomach contents was determined to be 3.87 and gas chromatography analysis revealed lactic acid levels of 151 mM. To determine the survival of S. Typhimurium in the presence of lactic acid, a synthetic gastric fluid (SGF) at pH 3.87 was utilized. A concentration and time-dependent lethal effect was observed on the survival of S. Typhimurium in SGF containing lactic acid. At 60 minutes, survival of wild-type S. Typhimurium decreased 500-fold in SGF containing 10 mM lactic acid compared to no decrease in survival without lactic acid. Compared to the wild-type strain, survival the of phoP, fur, and rpoS mutants in SGF containing 10 mM lactic acid was decreased 100, 500, and 1,000-fold, respectively. Additionally, a pnp mutant, which was previously identified by signature-tagged mutagenesis due to its sensitivity to challenge in the swine stomach contents assay, was also 1,000-fold more sensitive to challenge in SGF containing 10 mM lactic acid compared to the parent strain. The pnp gene encodes polynucleotide phosphorylase and is a component of the mRNA degradosome. A decrease in RpoS-regulated transcripts has been reported in a pnp mutant of E. coli and may indicate that the sensitivity of the pnp mutant is due to altered transcription of RpoS-regulated genes. The results of this research imply that the porcine gastric environment is composed of multiple components including low pH and organic acids that, when combined, decrease microbial survival during passage through the stomach.