Molecular aspects of acid resistance in food-borne bacterial pathogens: cues from Escherichia coli and Listeria monocytogenes.

The pH is one of the most important environmental factors capable of conditioning bacterial lifestyle. As a consequence of this, bacteria have developed a complex mechanism of regulation of the internal pH in response to variations of the external pH. The response occurs through the rapid switch of specific groups of genes. The activation or repression of these genes is mediated by sophisticated regulatory systems, and has the aim of maintaining a pH homeostasis by either passive or active mechanisms. Pathogenic bacteria are able to perceive and promptly respond to physical and chemical changes encountered during the passage from the environment to the host, and the low pH has, in particular, a profound impact on the colonising ability and virulence of enteric pathogens. Food-borne bacterial pathogens, including Escherichia coli and Listeria monocytogenes, are faced with proton attack at various stages during intestinal colonisation and/or infection. When orally ingested these bacteria persist within the stomach at pH<3 for ca. 2 hours before moving to the gut. Then, in the moderately acidic intestinal environment, they must counteract the deleterious effects of volatile fatty acids produced by sugar fermentation. In E. coli, the most efficient response to the acidic stress is provided by the inducible acid resistance (AR) systems, which protect the organism against a number of secondary environmental stressors. Three AR systems have been identified in E. coli, all of which are maximally expressed during the stationary phase of growth. The first system relies only on the stationary phase sigma factor RpoS, while the other two systems are based on the production of specific decarboxylases. The critical role of amino acid decarboxylases in the response to the acidic stress is inferred by their redundancy in enteric pathogens. E. coli produces two different isoforms of glutamic acid decarboxylase (GadA and GadB) and a glutamate/γ-amino butyrate antiporter protein (GadC). The AR systems are also major determinants of the infectious ability of some E. coli pathovars. This is the case of EHEC, which are capable of surviving in fermented foods and in the gastric juice of cattle by virtue of their intrinsic acid resistance, and EPEC, in which expression of gad and virulence genes is differentially regulated in a pH-dependent manner. Three different genes encoding putative Gad isozymes have been identified in L. monocytogenes. Two of these (gadA and gadB'C') show a similar arrangement as the E. coli homologues, while the third cluster (gadCB) has an inverted orientation. A link between the acid tolerance response (ATR) and virulence has also been proposed in L. monocytogenes. Acid tolerant mutants showed an increased virulence in mice and higher survival in both enterocyte-like cells and activated murine macrophages. The enhanced survival in macrophages has been correlated with an altered pattern of expression of some functions required for successful in vivo multiplication, including acid resistance, response to the oxidative stress and intracellular life.