The global trend toward intensive livestock production has led to significant public health risks and industry-associated losses due to an increased incidence of disease and contamination of livestock-derived food products. A potential factor contributing to these health concerns is the prospect that selective pressure within a particular host may give rise to bacterial strain variants that exhibit enhanced fitness in the present host relative to that in the parental host from which the strain was derived. Here, we assessed 184 Salmonella enterica human and animal clinical isolates for their virulence capacities in mice and for the presence of the Salmonella virulence plasmid encoding the SpvB actin cytotoxin required for systemic survival and Pef fimbriae, implicated in adherence to the murine intestinal epithelium. All (21 of 21) serovar Typhimurium clinical isolates derived from animals were virulent in mice, whereas many (16 of 41) serovar Typhimurium isolates derived from human salmonellosis patients lacked this capacity. Additionally, many (10 of 29) serovar Typhimurium isolates derived from gastroenteritis patients did not possess the Salmonella virulence plasmid, in contrast to all animal and human bacteremia isolates tested. Lastly, among serovar Typhimurium isolates that harbored the Salmonella virulence plasmid, 6 of 31 derived from human salmonellosis patients were avirulent in mice, which is in contrast to the virulent phenotype exhibited by all the animal isolates examined. These studies suggest that Salmonella isolates derived from human salmonellosis patients are distinct from those of animal origin. The characterization of these bacterial strain variants may provide insight into their relative pathogenicities as well as into the development of treatment and prophylactic strategies for salmonellosis.Salmonella enterica is a significant food-borne pathogen of humans transmitted via the consumption of meat, animal products, and food products (e.g., fruits and vegetables) contaminated with animal waste (24,41,76). Clinical manifestations of human and animal salmonellosis range from self-limiting gastroenteritis to severe bacteremia and typhoid fever. More than 2,300 serovars of S. enterica have been identified and classified previously, typically by serotyping based on antigenic variation in the lipopolysaccharide (O-antigen) and phase 1 (H1) and phase 2 (H2) flagella (22,45,46). Although serotyping has been an epidemiologically useful resource for classifying isolates, it provides relatively limited information regarding bacterial diversity, evolutionary relatedness, and pathogenicity (11,14,18,30,57,61).There are six subspecies of S. enterica, and the vast majority of human and animal infections are caused by strains belonging to subspecies I. In spite of the close genetic relationship among serovars assigned to subspecies I, there are significant differences in virulence, host adaptation, and host specificity (70, 71), and accordingly, they have been categorized into three different groups: broad...

Lysosomal acid a-glucosidase (EC 3.2

Comparative genomic analysis has revealed limited strain diversity between Salmonella pathogenic and nonpathogenic isolates. Thus, some of the relative virulence and host-immune response disparities may be credited to differential gene regulation rather than gross differences in genomic content. Here we show that altered levels of Salmonella DNA adenine methylase (Dam) resulted in acute defects in virulence-associated gene expression, motility, flagellin synthesis, and bile resistance in the Salmonella pathogenic strain 14028 but not in avirulent laboratory strain LT2. The defects in motility exhibited by 14028 in response to altered Dam levels was not dependent on the presence of the regulatory protein, RpoS. The transitioning between flagellar types (phase variation) was also differentially regulated in 14028 versus LT2 in response to dam levels, resulting in distinct differences in flagellin expression states. These data suggest that differential gene regulation may contribute to the relative virulence disparities observed between Salmonella serovars that are closely related at the DNA level.

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Salmonella enterica serovar Typhimurium that lacks the DNA adenine methylase (Dam) ectopically expresses multiple genes that are preferentially expressed during infection, is attenuated for virulence, and confers heightened immunity in vaccinated hosts. The safety of dam mutant Salmonella vaccines was evaluated by screening within infected mice for isolates that have an increased capacity to cause disease relative to the attenuated parental strain. Since dam mutant strains are sensitive to the DNA base analog 2-aminopurine (2-AP), we screened for 2-AP-resistant (2-AP r ) isolates in systemic tissues of mice infected with dam mutant Salmonella. Such 2-AP r derivatives were isolated following intraperitoneal but not oral administration and were shown to be competent for infectivity via intraperitoneal but not oral infection of naïve mice. These 2-AP r derivatives were deficient in methyl-directed mismatch repair and were resistant to nitric oxide, yet they retained the bile-sensitive phenotype of the parental dam mutant strain. Additionally, introduction of a mutH null mutation into dam mutant cells suppressed the inherent defects in intraperitoneal infectivity and nitric oxide resistance, as well as overexpression of SpvB, an actin cytotoxin required for Salmonella systemic survival. These data suggest that restoration of intraperitoneal virulence of dam mutant strains is associated with deficiencies in methyl-directed mismatch repair that correlate with the production of systemically related virulence functions.