Effects of Sodium Chloride or Calcium Chloride Concentration on the Growth and Survival of Escherichia coli O157:H7 in Model Vegetable Fermentations

Abstract

ABSTRACTSalt 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 (CaCl2) salt conditions. CaCl2 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% CaCl2, 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