Abstract
Numerous gene expression and stress adaptation responses in L. monocytogenes are regulated through alternative sigma factors σB and σL. Stress response phenotypes and transcriptomes were compared between L. monocytogenes EGD-e and its ΔsigB and ΔsigBL mutants. Targeted growth phenotypic analysis revealed that the ΔsigB and ΔsigBL mutants are impaired during growth under cold and organic-acid stress conditions. Phenotypic microarrays revealed increased sensitivity in both mutants to various antimicrobial compounds. Genes de-regulated in these two mutants were identified by genome-wide transcriptome analysis during exponential growth in BHI. The ΔsigB and ΔsigBL strains repressed 198 and 254 genes, respectively, compared to the parent EGD-e strain at 3 °C, whereas 86 and 139 genes, respectively, were repressed in these mutants during growth at 37 °C. Genes repressed in these mutants are involved in various cellular functions including transcription regulation, energy metabolism and nutrient transport functions, and viral-associated processes. Exposure to cold stress induced a significant increase in σB and σL co-dependent genes of L. monocytogenes EGD-e since most (62%) of the down-regulated genes uncovered at 3 °C were detected in the ΔsigBL double-deletion mutant but not in ΔsigB or ΔsigL single-deletion mutants. Overall, the current study provides an expanded insight into σB and σL phenotypic roles and functional interactions in L. monocytogenes. Besides previously known σB- and σL-dependent genes, the transcriptomes defined in ΔsigB and ΔsigBL mutants reveal several new genes that are positively regulated by σB alone, as well as those co-regulated through σB- and σL-dependent mechanisms during L. monocytogenes growth under optimal and cold-stress temperature conditions.
Highlights
The Gram-positive, facultatively intracellular food-borne pathogen Listeria monocytogenes is the causative agent of serious food-borne illness, listeriosis
Growth phenotypes between ∆sigB and ∆sigBL mutants and their parental strain were compared. Both mutants were diminished during cold growth in defined minimal medium (DM) but not in brain heart infusion (BHI) compared to the EGD-e WT parent strain
The ∆sigBL mutant displayed an even more prolonged lag phase as well as a slower growth rate compared to the parent strain under cold stress (Table 1 and Figure S1)
Summary
The Gram-positive, facultatively intracellular food-borne pathogen Listeria monocytogenes is the causative agent of serious food-borne illness, listeriosis. Since L. monocytogenes is widespread in the environment and can be found in soil, water, and decaying vegetation, it can access raw materials used by the food industry causing a significant microbial control challenge in food-processing facilities [4–11]. Transcriptional regulation of genes essential for growth under stress conditions that the bacteria have encountered, is among the most central mechanisms. Bacteria can regulate the transcription of genes through alternative sigma factors which control the function and promoter selectivity of bacterial RNA polymerase. L. monocytogenes has four known alternative sigma factors: σB, σC, σH, and σL. ΣB is the most extensively characterized sigma factor and is regarded as the major stress response regulator of L. monocytogenes, while in many other pathogenic bacteria, σL (rpoN) is considered central to regulation of the bacterial cell exterior [13] In L. monocytogenes, sigma-70 family comprises σB, σC, σD, and σH, while σL is the only member in the sigma-54 family [12]. σB is the most extensively characterized sigma factor and is regarded as the major stress response regulator of L. monocytogenes, while in many other pathogenic bacteria, σL (rpoN) is considered central to regulation of the bacterial cell exterior [13]
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