Abstract/Summary

Listeria monocytogenes is a foodborne pathogen associated with high mortality rates. Its success is partially due to its ability to survive and grow in a wide range of stressful conditions, such as oxidative stress (OS). The role of the main transcriptional stress regulator, SigB, for OS was assessed. It was determined that the presence of sigB made the cells sensitive to OS, showing a decreased survival and impaired growth in the presence of hydrogen peroxide (H2O2) in stationary phase. This hyper-sensitivity was accompanied by a decreased catalase activity and kat expression in the WT strain, in comparison with the ∆sigB. SigB is known to be activated by acidic pH and visible light, which could make the cells even more sensitive to OS. It was determined that cells pre-exposed to low pH were less tolerant to a subsequent oxidative challenge. Visible light, however had a dual effect. Not only was it able to promote sigB activation but also lead to the production of reactive oxygen species (ROS) in sub-lethal concentrations. Light-derived ROS can generate an adaptive response, making the cells more resistant to a subsequent oxidative challenge, overcoming the detrimental effects of SigB activation. It was described that bacterial cells in exponential phase of growth showed very low levels of dissolved oxygen (DO) possibly affecting oxidative stress resistance and other phenotypic characteristics. To study the effect of these low oxygen levels, and uncouple the stage of growth from these micro-aerophilic conditions we aimed at increasing the levels of oxygen in the culture. To achieve this, L. monocytogenes was grown in a bio-reactor set-up with constant aeration and agitation. The DO was successfully increased and bacterial cells were significantly more resistant to OS, and showed significantly more catalase activity, in comparison with cells grown in the conventional set-up. Furthermore, we discovered a possible role for the GAD system in oxidative stress. We found that the presence of a well functional glutamate decarboxylase (GAD) system is important for a correct response against OS. Mutants lacking components of the GAD system were tested and ∆gadD3 in both EGD-e and 10403S, and ∆gadD1 in LO28, were found to be significantly impaired in its ability to grow and survive in H2O2. lt was proposed that the intracellular GAD system contributes to the maintenance of intracellular anti-oxidant molecules important for OS tolerance.