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SigB is the main stress gene regulator inListeria monocytogenes affecting the expression of more than 150 genes and thus contributing to multiple-stress resistance. Despite its clear role in most stresses, its role in oxidative stress is uncertain, as results accompanying the loss ofsigB range from hyperresistance to hypersensitivity. Previously, these differences have been attributed to strain variation. In this study, we show conclusively that unlike for all other stresses, loss ofsigB results in hyperresistance to H2 O2 (more than 8 log CFU ml−1 compared to the wild type) in aerobically grown stationary-phase cultures ofL. monocytogenes strains 10403S and EGD-e. Furthermore, growth at 30°C resulted in higher resistance to oxidative stress than that at 37°C. Oxidative stress resistance seemed to be higher with higher levels of oxygen. Under anaerobic conditions, the loss of SigB in 10403S did not affect survival against H2 O2 , while in EGD-e, it resulted in a sensitive phenotype. During exponential phase, minor differences occurred, and this result was expected due to the absence ofsigB transcription. Catalase tests were performed under all conditions, and stronger catalase results corresponded well with a higher survival rate, 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, reverse transcription-PCR (RT-PCR) showed no differences in transcription between the wild type and the ΔsigB mutant in various oxidative stress genes. Further investigation of the molecular mechanism behind this phenotype and its possible consequences for the overall phenotype ofL. monocytogenes are under way.IMPORTANCE SigB is the most important stress gene regulator inL. 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 ofL. monocytogenes , and it might help us explain the behavior of this organism in environments where oxidative stress is present.

applied and environmental microbiology

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