The alternative sigma factor sigma(L) of L. monocytogenes promotes growth under diverse environmental stresses

Abstract

Listeria monocytogenes can cause sporadic cases and outbreaks of serious illnesses due to consumption of contaminated food products The challenge of this food-borne pathogen can in part be attributed to its remarkable stress tolerance. L. monocytogenes organisms efficiently adapt and sometimes proliferate despite exposure to low temperatures, low pH and elevated salt (NaCl) concentrations. To gain insights into some of the underlying adaptive mechanisms, the contribution of alternative sigma factor σL to counter such environmental stresses was investigated by quantitative RT-PCR of sigL gene transcripts and phenotypic growth evaluation of a sigL deletion mutant. Moreover, transcripts of selected σL regulated genes (oppA, cspD and clpP) were quantified in cold and NaCl stress adapted cells of a ΔsigL and the wild type strain. We found that sigL transcripts are variably but significantly induced by exposure of L. monocytogenes to the three different growth stress conditions. The growth of the ΔsigL strain was significantly impaired in comparison to the wild type under all stress conditions applied. Additionally, transcripts of oppA were more than 50-fold reduced in cold adapted ΔsigL cells compared to the wild type cells. These results suggest an important role of σL dependent pathways in L. monocytogenes adaptation against low temperatures, low pH and elevated salt (NaCl) concentrations