Sodium chloride decreases the bacteriocidal effect of acid pH on Escherichia coli O157:H45

To reduce the risk of foodborne illness, many fermented and minimally processed foods rely on the “hurdle effect”, i.e. a combination of two or more inhibitory agents being more inhibitory than any of the agents alone. However, we have observed that such a combination of agents is not always more inhibitory to foodborne pathogens than one alone. In this paper, we show that a combination of NaCl and acid pH is less effective than acid pH alone in reducing the numbers of Escherichia coli O157:H45. The presence of sodium chloride reduces the bacteriocidal effect of lactic acid on logarithmically growing cells of E. coli O157:H45, with approximately 103-fold more survivors at pHo 4.2 when 4% NaCl was added to the medium. A similar protective effect was also seen with other organic acidulants and E. coli strains. The cytoplasmic pH (pHi) of cells in medium at pH 4.2 containing added salt was 5.8 which was 0.56 units higher than that of cells in the same medium without added salt. When the pHi of cells in medium without added salt was adjusted to the same value (5.8) by adding KOH, the rate of survival was also considerably greater than that of cells in medium without added salt. These data suggest that E. coli can use NaCl to counteract acidification of its cytoplasm by organic acids, and in addition, that combinations of antimicrobial agents cannot always be relied upon to achieve additive antimicrobial effects.