Non-protective Role of sigB against Oxidative Stress in Listeria monocytogenes

Introduction: L. monocytogenes is a foodborne pathogen causing listeriosis, a life-threatening disease, that has to cope with extreme conditions, such as oxidizing environments, in order to survive and cause disease. Oxidative stress occurs during aerobic respiration, disinfection regimes (oxidative disinfectants) and during the infectious process. SigB is a well-known transcriptional regulator involved in regulating the expression of numerous stress- and virulence-related genes, whose role in oxidative stress is still controversial. 

Purpose: To clarify the role of sigB against oxidative stress during different stages of the cell cycle.

Methods: L. monocytogenes 10403S, wild type (WT) and sigB mutant (ΔsigB), were challenged with H₂O₂ in mid-exponential and stationary phases of growth. Bacterial viability was assessed by quantifying the colony forming units during the 60 minutes of challenge. In parallel, the levels of dissolved oxygen (DO) and the catalase activity were determined using a novel quantitative method.

Results: During stationary phase, the sigB mutant was significantly more resistant to H₂O₂ than the WT strain (~5 logs difference, p<0.05), despite both strains presenting similar DO levels (6.41±0.12 for WT and 6.50±0.22 for ΔsigB). However, during mid-exponential phase both WT and sigB mutant showed similar resistance to oxidative stress (~4logs reduction after 60 min of challenge). Interestingly, the levels of DO during mid-exponential phase were significantly lower (~0 mg/L) for both WT and sigB mutant. During mid-exponential phase the catalase activity was very low both in WT and ΔsigB strains, however after 13/14 hours of growth, the ΔsigB presented stronger catalase activity thant the WT.

Significance: We hypothesise that the presence of sigB gene has a non-protective effect against oxidative stress, which is probably mediated by the presence of oxygen. These findings will help us understand in depth the oxidative stress resistance mechanisms of this pathogen and thus reduce the occurrence of disease.