Effects of Organic Acids and Atmosphere on the Survival of Escherichia coli O157:H7 Under Conditions Similar to Acidified Foods

Abstract

The ability of Escherichia coli O157:H7 to survive in acidified vegetable products is of concern because of previously documented outbreaks associated with fruit juices. A study was conducted to determine the survival of E. coli O157:H7 in organic acids at pH values typical of acidified vegetable products (pH 3.2 and 3.7), under different atmospheres, and a range of ionic strengths (0.086 to 1.14). Determination of internal pH and catalase activity in acid solutions can explain how dissolved oxygen or D- versus L-lactic acid reduces the survival of Escherichia coli O157:H7 when the organism is exposed to low pH (3.2) values typical of fermented and acidified foods. All solutions contained 20 mM gluconic acid, which was used as a non-inhibitory low pH buffer to compare the individual acid effect to that of pH alone on the survival of E. coli O157:H7. E. coli O157:H7 cells challenged in buffered solution with ca. 5 mg/L dissolved oxygen (present in tap water) over a range of ionic strengths at pH 3.2 exhibited a decrease in survival as the ionic strength was increased over 6 h. Overall, under oxygen limiting conditions in an anaerobic chamber, there is no significant difference in the loss of viability of E. coli O157:H7 cells regardless of pH, acid type, concentration, or ionic strength. For lactate concentrations up to 40 mM, there was no significant difference in internal pH values when cells were incubated in D- versus L-lactic acid solutions. Unexpectedly, cells incubated under aerobic conditions maintained a significantly higher internal pH (ca. 5.8) than cells incubated under anaerobic conditions (ca. 5.4), regardless of the isomer of lactic acid used. Overall, catalase activity was higher when cells were in the presence of L-lactic acid versus D-lactic acid however there is a decrease in survival when cells are incubated in L-lactic acid. Under the conditions tested, differences in survival of E. coli O157:H7 between isomers of lactic acid and atmospheres is not a result of internal pH or catalase activity. However, the lower internal pH maintained by cells incubated anaerobically (5 mM lactic acid) results in a smaller pH gradient and decreases the accumulation of acid anions inside the cell, which may contribute to increased survival under anaerobic conditions. Many acid and acidified foods are sold in hermetically sealed containers with oxygen limiting conditions. Our results demonstrate that E. coli O157:H7 may survive better than previously expected from studies with acid solutions containing dissolved oxygen.