The role of the alternative sigma factor, sigma(B), in stress response and regulation of virulence in Listeria monocytogenes

The ability of the foodborne pathogen Listeria monocytogenes to grow in adverse conditions is well documented, however, the mechanism by which L. monocytogenes is capable of subverting the harmful effects of environmental stress is not well understood. In the related organism, Bacillus subtilis, the general stress response, controlled by the alternative sigma factor, σB, consists of a large regulon of genes whose products are synthesized in response to stress, promoting growth and survival in these conditions. The objective of this research was to determine whether the alternative sigma factor, σB, contributes to the adaptive physiology and virulence of L. monocytogenes. Additionally, the mechanism by which stress is communicated to σ B in L. monocytogenes was investigated. Sequence analysis demonstrated that σB was present in L. monocytogenes, along with an eight-gene operon that regulates its activity. Using primer extension analysis it was determined that various types of stress, including exposure to NaCl and shifts to high or low temperature, induce σB activity in L. monocytogenes. Loss of σB led to reduced growth rates in media containing high NaCl concentrations or when cells were incubated at low temperatures. Since it was known that L. monocytogenes responds to osmotic and low temperature shocks by accumulating the compatible solutes betaine and carnitine, we studied whether transport of these molecules was influenced by σB. The absence of σ B resulted in impaired accumulation of both betaine and carnitine. Loss of σB coincided with survival defects after exposure to low pH, hydrogen peroxide and nitric oxide. Macrophage invasion assays determined that a sigB null mutant was more sensitive to killing by these cells than the wild type strain, suggesting that σ B contributes to virulence of L. monocytogenes. Primer extension analysis detected σB activity after environmental stress in a strain containing a mutation in ribosomal protein L11 suggesting that the mechanism of stress communication to σB is independent of ribosome function in L. monocytogenes