Transcriptomic responses of Salmonella enterica serovars Enteritidis and Typhimurium to chlorine-based oxidative stress

Abstract

Salmonella enterica serovars Enteritidis and Typhimurium are the leading causative agents of salmonellosis in the United States. S. Enteritidis is predominantly associated with contamination of shell eggs and egg products, whereas S. Typhimurium is frequently linked to tainted poultry meats, fresh produce, and recently, peanut-based products. Chlorine is an oxidative disinfectant commonly used in the food industry to sanitize the surfaces of foods and food processing facilities (e.g., shell eggs and poultry meats). However, chlorine disinfection is not always effective, as some S. enterica strains may resist and survive the disinfection process. To date, little is known about the underlying mechanisms of how S. enterica responds to chlorine-based oxidative stress. In this study, we designed a custom bigenome microarray that consists of 385,000 60-mer oligonucleotide probes and targets 4,793 unique gene features in the genomes of S. Enteritidis strain PT4 and S. Typhimurium strain LT2. We explored the transcriptomic responses of both strains to two different chlorine treatments (130 ppm of chlorine for 30 min and 390 ppm of chlorine for 10 min) in brain heart infusion broth. We identified 209 S. enterica core genes associated with Fe-S cluster assembly, cysteine biosynthesis, stress response, ribosome formation, biofilm formation, and energy metabolism that were differentially expressed (>1.5-fold; P < 0.05). In addition, we found that serovars Enteriditis and Typhimurium differed in the responses of 33 stress-related genes and 19 virulence-associated genes to the chlorine stress. Findings from this study suggest that the oxidative-stress response may render S. enterica resistant or susceptible to certain types of environmental stresses, which in turn promotes the development of more effective hurdle interventions to reduce the risk of S. enterica contamination in the food supply.

FIG. 1.

Circular maps that compare the global gene expression profiles of S. Typhimurium strain LT2 and S. Enteritidis strain PT4 under the chlorine treatments. The innermost circle gives the genomic coordinates of the LT2 or PT4 chromosome. From the inside out, the second circle shows all protein-encoding genes color coded based on clusters of orthologous groups (COGs) of proteins (see the color codes at the bottom). The third and forth circles show the downregulated (blue) and upregulated (red) genes, respectively, under strong chlorine oxidation (390 ppm for 10 min). The fifth and sixth circles show the downregulated (blue) and upregulated (red) genes, respectively, under weak chlorine oxidation (130 ppm for 30 min). All differentially expressed genes are color coded based on the fold change relative to the control experiment (see the color code on the right side). Genes selected for quantitative RT-PCR are marked at the corresponding locations on the chromosome. The circular maps were constructed using the GenomeViz 1.2 software.

FIG. 2.

Hierarchical cluster plots that show the gene expression patterns of 33 stress-related genes (A) and 19 virulence-associated genes (B) in LT2 and PT4 under the weak (130 ppm) and strong (390 ppm) chlorine treatments. The pseudo-color bar indicates the relative fold change.

FIG. 3.

A bar graph that compares the gene expression fold changes identified by DNA microarrays and quantitative RT-PCR analysis. Six upregulated genes and six downregulated genes were compared. The fold changes were converted into log₂ values for comparison. Standard deviation error bars for each mean are presented. SE, S. Enteritidis PT4; ST, S. Typhimurium LT2; 30, the weak chlorine treatment (130 ppm for 30 min); 10, the strong chlorine treatment (390 ppm for 10 min); microarray, the fold change identified by microarray; RT-PCR, the fold change identified by qRT-PCR.