FIGURE 14.3 Regulatory network governing gadA/BC expression and glutamate-dependent acid resistance. (Adapted from Ma, Z., Gong, S., Richard, H., Tucker, D.L., Conway, T., and Foster, J.W., Mol. Microbiol., 49, 1309-1320, 2003. With permission.)
stresses other than acid [111-113]. A clear example of this crosstalk is exhibited by E. coli O157:H7 in which acid resistance is induced in response to heat stress , and heat tolerance is induced in response to acid stress . Crosstalk can be mediated by two-component (sensor-effector) regulatory systems used by bacteria, where sensor kinases phosphorylate noncognate regulatory proteins [116,117]. The precise nature of the signal(s) recognized by the cells for controlling acid resistance remains unclear, although considerable research has been carried out investigating genes induced by exposure to acid and other stresses.
Escherichia coli has several known inducible acid resistance systems that allow the organism to respond to the presence of organic acids and low pH in the environment [118,119]. The most well studied system uses decarboxylation of glutamic acid as a means for modulating internal pH . The system consists of two inducible proteins, glutamate decarboxylase (GadA and an isozyme GadB), and an antiport transporter (GadC) for glutamate and the decarboxylated product of glutamate, gamma-aminobutyric acid. The genetic regulation of this system has been found to be quite complex (Figure 14.3). RpoS, a sigma factor produced in response to stress, mediates expression of two regulatory proteins, GadW and GadX, that control expression of the decarboxylase and transport proteins . In addition, there is a two-component regulatory system that responds to (unidentified) external acid signals and can cause expression of the proteins of the glutamate decarboxylase system through the action of another regulatory protein, GadE [118,122]. The other acid resistance systems include arginine and lysine decarboxylase systems  similar to the glutamic acid system and a glucose-repressed, acid-induced system also controlled by RpoS which does not require external amino acids . Inducible acid resistance mechanisms have been observed in a variety of other food pathogens, including Salmonella spp., L. monocytogenes, Shigella flexneri, B. cereus, and others [111,123-126]. As the details of gene regulation of acid resistance of microbial food pathogens become clearer, strategies may be devised to help prevent the survival of these pathogens in acidified foods.
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