Comparison of the genetics and in-vitro virulence of Salmonella enterica serovar Typhimurium isolated from California dairy cattle and humans

The goal of this dissertation was to explore the possibility of genes responsible for differences in the host preference or specifity in strains of Salmonella enterica serovar Typhimurium (S. Typhimurium) isolated from cattle and humans. Initially, a variety of methods based on genotyping and phenotyping were used to cluster isolates from a library of 80 isolates. The methods included ribotyping, insertion sequence typing, pulsed-field gel electrophoresis (PFGE), plasmid typing, antibiotic sensitivity testing, and phage typing. The prevalence odds for human isolates being in one of the two largest clusters, relative to the second cluster, as defined by all methods except PFGE ranged from 1.4 to 2.5. In vitro assays of virulence, specifically adhesion and invasion, were used to determine if a subset of isolates differed in virulence.{09}In this system human isolates had statistically higher levels of adhesion to the epithelial cells than did bovine isolates but there were no differences in invasion. Lastly, subtractive hybridization was used in pairwise comparisons of human and bovine isolates in an effort to extract DNA regions present in one isolate of the pair but absent in the second. The DNA fragments identified were then used in a microarray to screen all isolates in the library for the presence or absence of those fragments. No single fragment had 100% presence in isolates from one host species and complete absence in isolates from the second host species. However, logistic regression using the signal intensity data from three fragments was found to have positive predictive values of 79% and 85% for identifying bovine and human isolates respectively.; This project suggests that there may be a degree of host specificity in some strains of S. Typhimurium but failed to identify any specific genetic region that would completely define such host specificity. The results indicate that the existence of regions encoding host specificity in this organism is possible and that further efforts to elucidate such regions are warranted.