Listeria monocytogenes Shows Temperature-Dependent and -Independent Responses to Salt Stress, Including Responses That Induce Cross-Protection against Other Stresses

The food-borne pathogen Listeria monocytogenes experiences osmotic stress in many habitats, including foods and the gastrointestinal tract of the host. During transmission, L. monocytogenes is likely to experience osmotic stress at different temperatures and may adapt to osmotic stress in a temperature-dependent manner. To understand the impact of temperature on the responses this pathogen uses to adapt to osmotic stress, we assessed genome-wide changes in the L. monocytogenes H7858 transcriptome during short-term and long-term adaptation to salt stress at 7 degrees C and 37 degrees C. At both temperatures, the short-term response to salt stress included increased transcript levels of sigB and SigB-regulated genes, as well as mrpABCDEFG, encoding a sodium/proton antiporter. This antiporter was found to play a role in adaptation to salt stress at both temperatures; Delta mrpABCDEFG had a significantly longer lag phase than the parent strain in BHI plus 6% NaCl at 7 degrees C and 37 degrees C. The short-term adaptation to salt stress at 7 degrees C included increased transcript levels of two genes encoding carboxypeptidases that modify peptidoglycan. These carboxypeptidases play a role in the short-term adaptation to salt stress only at 7 degrees C, where the deletion mutants had significantly different lag phases than the parent strain. Changes in the transcriptome at both temperatures suggested that exposure to salt stress could provide cross-protection to other stresses, including peroxide stress. Short-term exposure to salt stress significantly increased H2O2 resistance at both temperatures. These results provide information for the development of knowledge-based intervention methods against this pathogen, as well as provide insight into potential mechanisms of cross-protection.