## 2 Maintaining Intracellular pH under Acidic Conditions
monocytogenes_ and is responsible for the initiation of a signal transduction pathway that results in the release of \({}^{9}\)b and upregulation of the general stress response region (GSR). The arginine-dependent acid resistance system in _E. coli_ consists of the acid-inducible arginine decarboxylase (AdiA) and the arginine/agmatine antiporter (AdiC) that exchanges extracellular arginine for the intracellular decarboxylation product agmatine [29; 30; 31; 32] (Figure 1A). In _L. monocytogenes_, the ADI system consists of three enzymes and one membrane bound transporter: arginine deaminase (ArcA), catabolic ornithine carbamoyltransferase (ArcB, also known as AguB), carbamate kinase (ArcC, also known as AguC), and the arginine/ornithine antiporter ArcD (also known as AguD) [16] (Figure 1B). Arginine is converted into ornithine in a two-step reaction that also produces carbamoyl-phosphate and ammonia, which combines with intracellular protons to yield ammonium (NH4+), raising the cytoplasmic pH. Ornithine is transported out of the cell by the putative antiporter ArcD in exchange for a molecule of arginine. The _arcA_ gene mediates the acid response in vitro and contributes to survival in human gastric fluid [33]. The carbamoyl-phosphate is further metabolized by ArcC into ammonium and carbon dioxide with the production of ATP. This ATP may contribute to the extrusion of cytoplasmic protons by the F\({}_{0}\)F\({}_{1}\)-ATPase, another mechanism that has been shown to play a role in the maintenance of pH homeostasis [34]. A lesser-known mechanism of acid stress resistance in _L. monocytogenes_ is the agmatine deaminase (AgDI) pathway [17; 33; 35; 36] (Figure 1B). The ADI and AgDI systems share the same genetic locus and some of these genes are probably involved in both pathways. ArcB was the first enzyme to be described as having both ornithine and putrescine carbamoyltransferase activities [17] (Figure 1B). Two putative agmatine deaminase homologues are present in the _L. monocytogenes_ genome, namely _agua1_ and _agua2_, but only AguA1 has been shown to have AgDI activity [35]. These enzymes catalyze the transformation of agmatine into putrescine, ammonium, carbon dioxide, and ATP, in a series of reactions that are analogous to those described for the ADI system. Putrescine is then exchanged with agmatine via the putative ArcD antiporter, but the activity and substrate specificity of this transporter remains to be determined. Under mild acid stress (external pH -5), CadA catalyzes the proton-consuming decarboxylation of lysine in _E. coli_, producing CO\({}_{2}\) and the polyamine cadaverine, which is transported outside of the cell by the lysine/cadaverine antiporter CadB [18; 37; 38] (Figure 1A). The ornithine-dependent system, consisting of the ornithine decarboxylase SpeF and the ornithine/putrescine antiporter PotE, may also play a role under similar conditions, but its contribution to acid resistance in _E. coli_ has not been unequivocally defined as for the other decarboxylalases [19; 39]. Another proton consuming process in _L. monocytogenes_ is the production of acetoin from pyruvate. \(\alpha\)-Acetolactone synthase (ALS) condenses two molecules of pyruvate to produce \(\alpha\)-acetolactone, while consuming a proton. \(\alpha\)-Acetolactone decarboxylase (ALD) decarboxylalates acetolactone to form acetoin with the uptake of another proton [20] (Figure 1B). Expression of _alsS_ and _alsD_ is upregulated [40] and acetoin production rate is higher during acid stress [41]. This pathway also explains the role of thiamine in acid resistance in _L. monocytogenes_, since the enzymes involved in the conversion of pyruvate to acetoin both rely on this vitamin as a cofactor [42]. Mechanisms other than decarboxylation also play an important role in acid stress resistance. The F\({}_{0}\)F\({}_{1}\)-ATPase enzyme complex plays a major role in the regulation of intracellular pH in a number of bacteria [9], and the contribution of this proton translocating ATPase to acid resistance has also been studied in _E. coli_ and _L.