Escherichia coli and Salmonella enterica Are Protected against Acetic Acid, but Not Hydrochloric Acid, by Hypertonicity

Abstract

) demonstrated that an increased NaCl concentration prolongs survival of Escherichia coli O157 SERL 2 in a broth model simulating the aqueous phase of a food dressing or sauce containing acetic acid. We examined the responses of five other E. coli strains and four Salmonella enterica strains to increasing concentrations of NaCl under conditions of lethal acidity and observed that the average "lag" time prior to inactivation decreases in the presence of hydrochloric acid but not in the presence of acetic acid. For E. coli in the presence of acetic acid, the lag time increased with increasing NaCl concentrations up to 2 to 4% at pH 4.0, up to 4 to 6% at pH 3.8, and up to 4 to 7% (wt/wt of water) NaCl at pH 3.6. Salmonella was inactivated more rapidly by combined acetic acid and NaCl stresses than E. coli, but increasing NaCl concentrations still decreased the lag time prior to inactivation in the presence of acetic acid; at pH 4.0 up to 1 to 4% NaCl was protective, and at pH 3.8 up to 1 to 2% NaCl delayed the onset of inactivation. Sublethal injury kinetics suggest that this complex response is a balance between the lethal effects of acetic acid, against which NaCl is apparently protective, and the lethal effects of the NaCl itself. Compared against 3% NaCl, 10% (wt/wt of water) sucrose with 0.5% NaCl (which has similar osmotic potential) was found to be equally protective against adverse acetic acid conditions. We propose that hypertonicity may directly affect the rate of diffusion of acetic acid into cells and hence cell survival.We previously observed that inactivation of Escherichia coli O157 SERL 2 by acetic acid at adverse pH in a broth model simulating the aqueous phase of acidic sauces and dressings was reduced by the presence of NaCl (4). Specifically, the time to a 3-log 10 -unit reduction (t 3D ) of E. coli SERL 2 as function of NaCl concentration was significantly nonmonotonic; that is, the t 3D initially increased when NaCl was increased (from 1 to 3% [wt/wt] of solution), but the t 3D decreased upon a further increase in NaCl concentration (to 8% [wt/wt] of solution) (4). The statistical significance of this "nonmonotonic" response increased with increasing exposure time from 24 to 72 h (at 23°C), primarily due to a proportionally greater increase in inactivation at 1% (wt/wt) NaCl with increasing treatment time than that which was observed at higher NaCl concentrations (4).The combination of acid and NaCl is a common example of the food industry's "hurdle" approach, which is used to preserve a large and diverse range of foods, including acidic dressings and sauces, fermented meats, cheeses, and preserved vegetables. Given the widespread use of this hurdle combination in food manufacturing, the first aim of this study was to determine whether the observed protection of E. coli SERL 2 from acid inactivation by NaCl is common among E. coli and Salmonella enterica and at what NaCl concentration maximum protection is achieved. A second aim was to determine whether NaCl protection is specific against acid...

Insights on Osmotic Tolerance Mechanisms in Escherichia coli Gained from an rpoC Mutation

“…Under higher osmotic stress with 0.7 M NaCl, acetic acid addition still had a benefit on the growth of BW25113, and showed a moderate benefit on the growth of EJW3 ( Figure 6 C). Prior work has shown that moderate concentration of NaCl can protect E. coli from acetic acid toxicity [ 48 ]; here we show that the cross tolerance is reciprocal, that moderate concentrations of acetic acid can also protect E. coli from NaCl stress. Interestingly, a statistically significant overlap ( p < 10 −20 , Fisher’s exact test) between metabolites level changes in organic acid and osmotolerant E. coli mutants has been detected using an updated version of the Resistome combined with a recent genome-wide survey of genotype–metabolite relationships [ 49 , 50 ], providing additional evidence that the improved osmotic tolerance of EJW3 may also result from the higher production of acetic acid.…”

Reduction in Microbial Load of Wheat by Tempering with Organic Acid and Saline Solutions

“…Bacteria may be able to maintain intracellular pH in the presence of NaCl by coupling the import of Na + ions to the H + export (Casey and Condon 2002). Additionally, an increase in osmotic pressure may induce a slightly plasmolyzed physiological state; thus, diffusion of protonized acetic acid into the cytoplasm may be slowed by a more rigid cell membrane (Chapman and Ross 2009).…”

Phytic Acid and Sodium Chloride Show Marked Synergistic Bactericidal Effects against Nonadapted and Acid-Adapted Escherichia coli O157:H7 Strains

“…In contrast, the combination of other organic acids plus NaCl had no effect under the same experimental conditions. Previous studies examining the antimicrobial relationship between organic acids and NaCl mainly used long exposure times (0.5 to 120 h) along with acetic or lactic acid (pH 3.0 to 4.2) and 2 to 8% NaCl in nutrient broth or TSB to model acidic foods (e.g., dressings, fermented foods, pickled vegetables, and sauces) (24)(25)(26)(27)(28)(29). They all concluded that NaCl (3 to 5%) protects cells from the antimicrobial effects of organic acids.…”

“…The combination of organic acids and NaCl is a common example of hurdle technology in the food industry; many researchers have found that NaCl (at 3 to 5%) protects bacterial cells from the antimicrobial effects of organic acids (an-tagonism) (24)(25)(26)(27)(28)(29)(30). However, it is unclear how PA interacts with NaCl.…”