Abstract
Listeria monocytogenes is a saprophytic bacterium that thrives in diverse environments and causes listeriosis via ingestion of contaminated food. RsbX, a putative sigma B (σ(B)) regulator, is thought to maintain the ready state in the absence of stress and reset the bacterium to the initial state in the poststress stage in Bacillus subtilis. We wondered whether RsbX is functional in L. monocytogenes under different stress scenarios. Genetic deletion and complementation of the rsbX gene were combined with survival tests and transcriptional and translational analyses of σ(B) expression in response to stresses. We found that deletion of rsbX increased survival under secondary stress following recovery of growth after primary stress or following stationary-phase culturing. The ΔrsbX mutant had higher expression of σ(B) than its parent strain in the recovery stage following primary sodium stress and in stationary-phase cultures. Apparently, increased σ(B) expression had contributed to improved survival in the absence of RsbX. There were no significant differences in survival rates or σ(B) expression levels in response to primary stresses between the rsbX mutant and its parent strain during the exponential phase. Therefore, we provide clear evidence that RsbX is a negative regulator of L. monocytogenes σ(B) during the recovery period after a primary stress or in the stationary phase, thus affecting its survival under secondary stress.