Products of theEscherichia coliAcid Fitness Island Attenuate Metabolite Stress at Extremely Low pH and Mediate a Cell Density-Dependent Acid Resistance

Abstract

Escherichia coli has an ability, rare among the Enterobacteriaceae, to survive extreme acid stress under various host (e.g., human stomach) and nonhost (e.g., apple cider) conditions. Previous microarray studies have exposed a cluster of 12 genes at 79 centisomes collectively called an acid fitness island (AFI). Four AFI genes, gadA, gadX, gadW, and gadE, were already known to be involved in an acid resistance system that consumes an intracellular proton through the decarboxylation of glutamic acid. However, roles for the other eight AFI gene products were either unknown or subject to conflicting findings. Two new aspects of acid resistance are described that require participation of five of the remaining eight AFI genes. YhiF (a putative regulatory protein), lipoprotein Slp, and the periplasmic chaperone HdeA protected E. coli from organic acid metabolites produced during fermentation once the external pH was reduced to pH 2.5. HdeA appears to handle protein damage caused when protonated organic acids diffuse into the cell and dissociate, thereby decreasing internal pH. In contrast, YhiF-and Slp-dependent systems appear to counter the effects of the organic acids themselves, specifically succinate, lactate, and formate, but not acetate. A second phenomenon was defined by two other AFI genes, yhiD and hdeD, encoding putative membrane proteins. These proteins participate in an acid resistance mechanism exhibited only at high cell densities (>10 8 CFU per ml). Densitydependent acid resistance does not require any demonstrable secreted factor and may involve cell contactdependent activation. These findings further define the complex physiology of E. coli acid resistance.

Role of the Multidrug Resistance Regulator MarA in Global Regulation of the hdeAB Acid Resistance Operon in Escherichia coli

“…The bacterial strains and plasmids used in this study are listed in Table 1. Because HdeA and HdeB seem to be mainly important in rich medium (28), we used LB medium (per liter: 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl) at pH 7.0 or buffered to pH 5.5 with 100 mM MES (morpholinoethanesulfonic acid). All cultures were grown at 37°C with agitation.…”

“…hdeAB REPRESSION BY MarA 1291 have a major effect on the expression of hdeAB (exponential versus stationary phase and pH 7.0 versus pH 5.5 [19,39,43,44]) and on the physiology of the HdeA and HdeB proteins (LB rich medium [28]). In a ⌬marRAB strain bearing an hdeABp-lacZ chromosomal reporter, the expression of hdeAB at pH 7.0 increased 1.7-fold from exponential phase to early stationary phase (Fig.…”

“…Both chaperones are only active below pH 3 and prevent irreversible aggregation of acid-denatured periplasmic proteins (14,21,23,28). HdeA seems to play a major role at pH 2, whereas HdeB is more active at pH 3 (23).…”

Bacterial Genome Instability

“…Introduction of a new GI can result in a total change of phenotype, behavior, or life-style of the receiving organism. Depending on the provided phenotypic advantages, a GI can be a pathogenicity island (such as Salmonella pathogenicity island 1 [SPI1] [104]), a fitness island (such as E. coli acid fitness island [AFI] [105]), a metabolic island (such as the Xanthomonas xanthan gum production island [106]), a resistance island (to antibiotics) (such as AbaR7 in Acinetobacter baumannii [107]), a symbiosis island (such as the Mesorhizobium loti strain R7A symbiosis island [108]), a saprophytic island (like the island encoding adhesins in some E. coli strains [94]), an ecological island (such as an island permitting phenol degradation in Pseudomonas putida [94,109]), or a defense island (as in Shewanella sp. strain ANA-3 [110]).…”

Comparative proteomics reveal distinct chaperone–client interactions in supporting bacterial acid resistance

“…Noteworthy, the HdeA and HdeB genes are regulated by a single promoter (7) and are well-conserved in various serotypes and pathotypes of E. coli, Shigella flexneri, and Brucella abortus (2). This hdeAB operon is highly expressed in response to acid stress (8), whereas disruption of these genes renders such bacteria extremely vulnerable to acid (5,9,10).…”