Protective effects of organic acids on survival of Escherichia coli O157:H7 in acidic environments.
Outbreaks of disease due to acid-tolerant bacterial pathogens in apple cider and orange juice have raised questions about the safety of acidified foods. Using gluconic acid as a noninhibitory low-pH buffer, we investigated the killing of Escherichia coli O157:H7 strains in the presence or absence of selected organic acids (pH of 3.2), with ionic strength adjusted to 0.60 to 0.68. During a 6-h exposure period in buffered solution (pH 3.2), we found that a population of acid-adapted E. coli O157:H7 strains was reduced by 4 log cycles in the absence of added organic acids. Surprisingly, reduced lethality for E. coli O157:H7 was observed when low concentrations (5 mM) of fully protonated acetic, malic, or l-lactic acid were added. Only a 2- to 3-log reduction in cell counts was observed, instead of the 4-log reduction attributed to pH effects in the buffered solution. Higher concentrations of these acids at the same pH aided in the killing of the E. coli cells, resulting in a 6-log or greater reduction in cell numbers. No protective effect was observed when citric acid was added to the E. coli cells. d-Lactic acid had a greater protective effect than other acids at concentrations of 1 to 20 mM. Less than a 1-log decrease in cell numbers occurred during the 6-h exposure to pH 3.2. To our knowledge, this is the first report of the protective effect of organic acids on the survival of E. coli O15:H7 under low-pH conditions.
Bjornsdottir
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U.S. Department of Agriculutre, Agricultural Research Service, Raleigh, NC 27695-7624, USA.
Breidt
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McFeeters
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RF
D000085:Acetates / Q000494:pharmacology; D000818:Animals; D002264:Carboxylic Acids / Q000494:pharmacology*; D019343:Citric Acid / Q000494:pharmacology; D015169:Colony Count, Microbial; D019453:Escherichia coli O157 / Q000187:drug effects / Q000254:growth & development*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000494:pharmacology; D008293:Malates / Q000494:pharmacology
D000085:Acetates; D002264:Carboxylic Acids; D008293:Malates; D019343:Citric Acid; D019344:Lactic Acid; C030298:malic acid