Escherichia coli O157:H7 (STEC) acid resistance may aid the pathogenâs ability to cross the human gastric barrier and makes it an organism of concern in acidic foods. Our objective was to determine how STEC acid resistance may correlate with survival during vegetable fermentations. Seven E. coli O157:H7 strains were screened to assess acid resistance in simulated stomach-acid at pH 2. The strains were separated into two groups that differed in acid resistance (p < 0.05), with three being acid sensitive and four acid resistant. The growth rates of these strains were measured in a Luria broth at pH values from 4.2 to 6.8. Two strains having similar growth kinetics, B201 (acid sensitive) and B241 (acid resistant), were selected for further analysis. B201 was found to be missing (compared to B241) two glutamic acid decarboxylase regulatory genes required for acid resistance, gadE and gadX. These strains were challenged in lactic acid (100 mM) solutions including cucumber juice (CJ) media at pH 3.3. As expected, B201 was more acid sensitive than B241, and a filtered fermented CJ was more inhibitory than similarly acidified CJ. In competitive growth studies with Lactobacillus plantarum LA445 in CJ, B201 or B241 grew from approximately 104 CFU/mL to 108 CFU/mL within 24 hours but the STEC strains were below the limit of detection by 48 hours. In all fermentations L. plantarum reached 108 CFU/mL by 48 hours. However, in three out of four independent fermentation experiments, strain B201 survived longer than B241. This was possibly due to buffering in B241-LA445 fermentation brines which had increased lactic acid for a given pH compared to B201-LA445. These data indicate that stationary phase acid resistance may not accurately predict STEC survival during vegetable fermentations.
ABSTRACT Salt concentration has long been considered an important factor for the quality of fermented vegetable products, but the role of salts in bacterial growth and death during vegetable fermentation remains unclear. We compared the effects of various sodium chloride (NaCl) concentrations, including 1 M (6%) NaCl used in commercial cucumber fermentations and 0.34 M (2%) NaCl used in cabbage and other ready-to-eat vegetable fermentations, on the growth and death of lactic acid bacteria (LAB) of the genus Lactobacillus and pathogenic Escherichia coli (Shiga toxin–producing E. coli, or STEC) strains. We also investigated calcium chloride (CaCl 2 ) salt conditions. CaCl 2 is being used at 0.1 M (1.1%) in low-salt commercial cucumber fermentations that lack added NaCl. STEC strains have previously been shown to be among the most acid-resistant pathogens in fermented or acidified vegetables. The data showed that 1.1% CaCl 2 , and especially 1% NaCl, had a stimulatory effect on the growth rates of STEC and LAB compared with a no-salt control, but higher NaCl concentrations decreased growth rates for STEC; to a lesser extent, LAB growth rates were also reduced. For most salt concentrations tested, maximum cell densities achieved during growth of STEC were reduced compared with those of the no-salt controls, whereas LAB mostly had cell densities that were similar to or greater than those of the no-salt controls. No consistent pattern was observed when comparing death rates with salt type or concentration for the STEC or LAB cocktails undergoing lactic acid stress (50 or 350 mM, respectively) at pH 3.2 and when comparing STEC survival in competitive culture experiments with LAB. For vegetable fermentation safety concerns, the results suggest that an important effect of salt addition is enhancement of the growth of LAB compared with STEC strains. Further research will be needed to determine factors influencing STEC survival in competition with LAB in vegetable fermentations. HIGHLIGHTS NaCl and CaCl 2 concentrations affected LAB and STEC strains differently. Growth rates at 6% NaCl were reduced for STEC more than LAB in vegetable broth. Extent of growth was reduced for STEC versus LAB for most vegetable fermentations. Death rates were minimally affected by salt type or concentration with lactic acid. Correlations between salt and STEC die-off were inconsistent for fermentation.