RNA polymerase sigma factor B: Evolution and role in environmental stress resistance in Listeria monocytogenes

The RNA polymerase alternative sigma factor B (σB) appears to play an important role in facilitating rapid adaptation to and survival in stressful environments in several Gram-positive bacteria. To investigate the role of σB in protecting L. monocytogenes from environmental stresses, survival of a sigB null mutant strain was compared with that of the wild type strain under environmental stress conditions. In stationary phase, L. monocytogenes resistance to heat (50°C) and ethanol (16.5%) stresses appears to beσ B independent, while resistance to oxidative (13.8 mM cumene hydroperoxide) and acid (pH 2.5) stresses appears to be σB dependent. σ B enhanced cell survival under conditions of carbon starvation and in the presence of organic acid (100 mM acetic acid) stress. L. monocytogenes acquires enhanced acid resistance upon entry into stationary phase, defined as the growth-phase dependent acid resistance (AR), and enhanced resistance to lethal acid following exposure to a milder acid stress, defined as the adaptive acid tolerance response (ATR). σB was shown to contribute to regulation of both mechanisms of acid resistance in L. monocytogenes and σB regulation of these mechanisms was shown to contribute to cell survival in synthetic gastric fluid (pH 2.5). σB regulation of proton flux across the cell membrane and of intracellular pH buffering was investigated to determine the σ B-directed-mechanisms for acid resistance. Under the conditions used in this study, neither proton flux across cell membranes or intracellular pH buffering were σB dependent. σB contributions to viability during carbon starvation, to acid and oxidative stress resistances and cell survival in synthetic gastric fluid support a role for σB in protecting L. monocytogenes against environmental adversities, which may include the defense barriers encountered by this pathogen in a host.; The sigB operon sequence for Listeria monocytogenes and for other Grampositive bacteria were used to probe the evolution and functional conservation of the σB operon among different bacteria. The σB operon appears to have diverged significantly in its overall components and the sequence of individual proteins, even in closely related bacterial species. Evolution of this stress response system appears to represent an important means for bacteria to respond and survive under a variety of stress conditions.