Loss of SigB in Listeria monocytogenes Strains EGD-e and 10403S Confers Hyperresistance to Hydrogen Peroxide in Stationary Phase under Aerobic Conditions

Marcia Boura, Ciara Keating, Kevin Royet, Ranju Paudyal, Beth O'Donoghue, Conor P. O'Byrne, Kimon A. G. Karatzas

SigB is the main stress gene regulator in L. monocytogenes affecting the expression of more than 150 genes and thus contributing in multiple stress resistance. Despite its clear role in most stresses, its role in oxidative stress is uncertain as results accompanying the loss of sigB range from hyperresistance to hypersensitivity. Previously, these differences have been attributed to strain variation. In this study, we show conclusively that in contrast to all other stresses, loss of sigB results in hyperresistance against H2O2 (more than 8 log CFU ml(-1) compared to the wild type) in aerobically-grown stationary phase cultures of 10403S and EGD-e.. Furthermore, growth at 30°C resulted in higher resistance to oxidative stress than at 37°C. Oxidative stress resistance seemed to be higher with higher levels of oxygen. Under anaerobic conditions, loss of SigB in 10403S did not affect survival against H2O2 while in EGD-e it resulted in a sensitive phenotype. During exponential phase, minor differences occurred as expected due to the absence of sigB transcription. Catalase tests were performed under all conditions and stronger catalase results corresponded well with higher survival underpinning the important role of catalase in this phenotype. Furthermore, we assessed the catalase activity in protein lysates which corresponded with the catalase tests and survival. In addition, RT-PCR showed no differences in transcription between the wild type and the ΔsigB in various oxidative stress genes. Further investigation of the molecular mechanism behind this phenotype and its possible consequences for the overall phenotype of L. monocytogenes are underway. SigB is the most important stress gene regulator in L. monocytogenes and other Gram positive bacteria. Its increased expression during stationary phase results in resistance to multiple stresses. However, despite its important role in general stress resistance its expression is detrimental for the cell in the presence of oxidative stress as it promotes hypersensitivity against hydrogen peroxide. This peculiar phenotype is an important element of the physiology of L. monocytogenes which could help us explain the behaviour of this organism in environments where oxidative stress is present.