Expression of the Virulence Gene hilA in Salmonella enterica is Suppressed by Lactobacillus casei Secondary Metabolites

Introduction: Salmonella spp. are the leading cause of foodborne illness among humans. The intestinal track of poultry can become colonized with Salmonella without causing harm or illness to the bird, which may lead to the potential contamination of poultry products during processing. Thus, pre-harvest intervention measures that aim to reduce colonization have the potential to reduce raw product contamination. Lactobacillus also colonizes the intestinal tract of poultry and has been utilized as a probiotic culture to eliminate Salmonella from the intestines of poultry. However, exactly how Lactobacillus eliminates Salmonella is not completely understood, but has been partially explained by the production of antimicrobial substances and by competing for niches and nutrients. We hypothesized that secondary metabolites produced by Lactobacillus may interfere with the expression of genes involved in colonization and virulence in Salmonella.

Purpose: Thus, the purpose of these experiments was to measure any changes in gene expression of Salmonella in response to exposure to secondary metabolites of Lactobacillus casei.

Methods: Salmonella enterica serovars Typhimurium (DT104) and Enteritidis (wild-type) and Lactobacillus casei (ATCC 334 and 393) were cultured in Man-Rogosa-Sharp broth (MRS). Lactobacillus cultures then were filter sterilized to produce sterile spent MRS medium and the pH was adjusted to either 6.2 or 5.5. Salmonella cultures were centrifuged and resuspended in either the spent or sterile, fresh MRS with a pH adjusted to 6.2 or 5.5. Total RNA then was extracted from these resuspended cultures at specific time points (0, 2, 4 and 24 hours) and the expression of hilA was evaluated using reverse transcriptase real-time PCR (RT-PCR).

Results: The hilA gene was upregulated in Salmonella when cultures were suspended in sterile, fresh medium with a slightly acidic pH. However, upregulation of the expression of hilA was suppressed in Salmonella cultures that were suspended in the sterile spent medium.

Significance: From these experiments, it appears that secondary metabolites produced by Lactobacillus casei suppress the upregulation of the virulence regulator hilA. Further work will be conducted to determine what metabolites are specifically involved in the suppression of the hilA gene. In addition, a global view of all genes under these conditions will be conducted utilizing a microarray approach to understand the full impact of the Lactobacillus secondary metabolites on Salmonella gene expression.