Comparative analysis of extreme acid survival in Salmonella typhimurium, Shigella flexneri, and Escherichia coli

Abstract

Several members of the family Enterobacteriaceae were examined for differences in extreme acid survival strategies. A surprising degree of variety was found between three related genera. The minimum growth pH of Salmonella typhimurium was shown to be significantly lower (pH 4.0) than that of either Escherichia coli (pH 4.4) or Shigella flexneri (pH 4.8), yet E. coli and S. flexneri both survive exposure to lower pH levels (2 to 2.5) than S. typhimurium (pH 3.0) in complex medium. S. typhimurium and E. coli but not S. flexneri expressed low-pHinducible log-phase and stationary-phase acid tolerance response (ATR) systems that function in minimal or complex medium to protect cells to pH 3.0. All of the organisms also expressed a pH-independent general stress resistance system that contributed to acid survival during stationary phase. E. coli and S. flexneri possessed several acid survival systems (termed acid resistance [AR]) that were not demonstrable in S. typhimurium. These additional AR systems protected cells to pH 2.5 and below but required supplementation of minimal medium for either induction or function. One acid-inducible AR system required oxidative growth in complex medium for expression but successfully protected cells to pH 2.5 in unsupplemented minimal medium, while two other AR systems important for fermentatively grown cells required the addition of either glutamate or arginine during pH 2.5 acid challenge. The arginine AR system was only observed in E. coli and required stationary-phase induction in acidified complex medium. The product of the adi locus, arginine decarboxylase, was responsible for arginine-based acid survival.

Development of a 3-hydroxypropionate resistantEscherichia colistrain

“…10 Although most AR systems are activated in stationary phase, the percent survival is still too low. 11 In industrial fermentations, 3HP production is usually performed under pH controlled conditions. Large volumes of base titrant are needed to maintain a constant pH, leaving the final acid molecule in the undissociated form.…”

The acid-resistance pathways of Shigella flexneri 2457T

“…GABA is then exchanged for external glutamate by the antiporter GadC, thereby maintaining the pH homeostasis of the cytoplasm, reversing the cell membrane potential to create an internal positive charge, and gradually alkalizing the extracellular medium (Castanie-Cornet & Foster, 2001;Richard & Foster, 2004). Thus, this pathway is dependent on glutamate being present in the acid-shock media (Lin et al, 1995). A number of genes involved in regulating this pathway have been identified in E. coli, revealing a complex network of regulation (Hommais et al, 2004;Masuda & Church, 2003).…”

“…The GDAR pathway, which is induced by growth in mildly acidic conditions (pH 5) and in fermentatively grown cells, is a glutamate decarboxylase system consisting of two homologous decarboxylase enzymes, gadA and gadB, and an antiporter, gadC (Hersh et al, 1996;Lin et al, 1995;Waterman & Small, 1996). This system appears to act by mopping up protons leaking into the bacterial cytosol through the decarboxylation of glutamate to gammaaminobutyric acid (GABA).…”

“…However, in order to do so, S. flexneri must pass through the acid environment of the human stomach, which can be at pH,3 (Giannella et al, 1972). Additionally, enteric bacteria must survive low-pH stress external to the host, such as that encountered during passage through the environment, in acidic foods and in fermenting acidified faecal material (Bhagwat & Bhagwat, 2004;Lin et al, 1995). The ability of S. flexneri to survive these low pH levels appears to be an important pathogenic characteristic, and may be a contributing factor to the low infectious dose of 10-100 organisms required for shigellosis to occur (Gorden & Small, 1993).…”

“…Studies have shown that S. flexneri possesses at least two acid-resistance pathways that seem to be similar to two of the three pathways identified in E. coli. Acid-resistance pathway 1 (AR1) is a stationary-phase, acid-induced, glucose-repressed oxidative pathway, while acid-resistance pathway 2 (AR2) is a stationary-phase, glutamate-dependent acid-resistance (GDAR) pathway (Lin et al, 1995).…”