Prevalence and Genetics of Survival of Salmonella in Oysters

Salmonella enterica is a leading cause of food-borne gastrointestinal disease worldwide. A survey conducted in 2002-2003 showed that oysters may contain Salmonella and thus may also be a source of salmonellosis. Since oysters are commonly consumed raw, no amount of food safety education will prevent consumers from ingesting a possibly infectious dose from Salmonella contaminated oysters. The research in this dissertation employed a combination of traditional culture techniques as well as genomics-based molecular applications to explore Salmonella infection in oysters and the subsequent risk to consumers of raw oysters. A year-long survey of oysters served on the half-shell in local restaurants determined that overall 1.2% of oysters were contaminated with Salmonella. Oysters containing Salmonella were found in 7 of the 8 months surveyed and 7 of the 8 restaurants served contaminated oysters. Six different serovars were isolated, but one strain of S. Newport, as determined by matching pulsed field gel electrophoresis patterns, represented 43% of the positive samples. Interestingly, this is the same strain that was predominantly isolated in the earlier survey of oysters and was also resistant to at least 7 different antimicrobials. The remainder of this dissertation work was an exploration of why this particular strain is seen so often in oyster infections. A custom microarray was used to perform a transposon site hybridization (TraSH) assay to identify genes that are necessary for S. Newport survival in the oyster. In this way, a negative selection was able to determine the genes that were necessary for S. Newport to survive in oysters. A subset of the genes identified by TraSH was selected and site-directed mutagenesis was performed to knock those genes out of LAJ160311. Oysters were infected with those mutant strains to test for their ability to survive in oysters and thereby determine the role of those individual genes in pathogenesis. The conclusions of the TraSH assay were that virulence factors that are essential for survival of Salmonella in mammalian models, particularly the type three secretion systems, may not be important in the oyster model. Motility provided by flagella was identified as a major virulence factor in oyster colonization by S. Newport