Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2011
Publication Date: 4/1/2012
Citation: Joerger, R.D., Sartori, C., Frye, J.G., Turpin, J.B., Schmidt, C., Mcclelland, M., Porwollik, S. 2012. Gene expression analysis of Salmonella enterica Enteritidis NalR and Salmonella enterica Kentucky 3795 exposed to HCl and acetic acid in rich medium. Foodborne Pathogens and Disease. 9(4):331-337. Interpretive Summary: In the United States, Salmonella enterica serovar Kentucky has become one of the Salmonella most frequently isolated from chickens. While Kentucky is the most common type of Salmonella found in chickens the number of Salmonella serovar Enteritidis isolated form chickens varies from year to year. Interestingly, while neither serovar cause disease in poultry, Enteritidis appears to result in more human diseases while Kentucky only rarely causes disease in humans. The reasons for this are not well understood. Survival in acidic (low pH) conditions is necessary for Salmonella to colonize a host because of the low pH in animal digestive tracts. Studies of S. Kentucky isolated from chickens showed it responded differently to acid than other serovars. To explore this, gene expression of S. Kentucky was compared to S. Enteritidis. These strains were tested by ten minute exposures to HCl (pH 4.5) or to acetic acid (pH 5.5) in rich media. Microarray analysis indicated that more genes were turned on or off in S. Kentucky than in S. Enteritidis under these conditions. Both serovars appear to use the same lysine-based pH stabilization system during acid exposure. Overall, it appears that the responses to acid by S. Kentucky and S. Enteritidis are similar, but differences exist in the scope and magnitude of the responses. These responses could explain the difference in prevalence of serovar Kentucky and Enteritidis in chickens. Further investigations could identify host responses by the chickens that could be adjusted to reduce the levels of dangerous pathogens like S. Enteritidis in chickens, thus improving food safety and reducing the risk of infection to humans.
Technical Abstract: In the United States, serovar Kentucky has become one of the most frequently isolated Salmonella enterica serovars from chickens. The reasons for this prevalence are not well understood. Phenotypic comparisons of poultry Salmonella isolates belonging to various serovars demonstrated that serovar Kentucky isolates differed from those of most other serovars in their response to acid. Gene expression analysis was performed with aerated exponentially growing poultry isolates, Salmonella enterica serovar Kentucky 3795 and Enteritidis NalR, exposed for 10 min to tryptic soy broth adjusted to pH 4.5 with HCl and to pH 5.5 with HCl or acetic acid. Data obtained by microarray analysis indicated that more genes were up- or down-regulated in strain Kentucky 3795 than in Enteritidis NalR under acidic conditions. Acid exposure in general caused up-regulation of energy metabolism genes and down-regulation of protein synthesis genes, particularly of ribosomal protein genes. Both strains appear to similarly utilize the lysine-based pH homeostasis system as up-regulation of cadB was observed under the acidic conditions. Expression of regulatory genes (rpoS, fur, phoPQ) known to be involved in the acid response showed similar trends in both isolates. Differences between Kentucky 3795 and Enteritidis NalR were observed with respect to the expression of the hdeB-like locus SEN1493 (potentially encoding a chaperone important to acid response), and some differences in the expression of other genes such as those involved in citrate utilization and motility were noted. qPCR analysis data generally paralleled the microarray data. It appears that the early stages of the transcriptional response to acid by isolates Kentucky 3795 and Enteritidis NalR are similar, but differences exist in the scope and in some facets of the response. It is possible that more prolonged exposure to HCl or acetic acid might reveal more distinct differences in gene expression or that the quantitative differences observed might lead to differences in protein levels that could explain the observed differences 36 in the acid phenotype of serovar Kentucky and other Salmonella serovars.