Effects of pH, dissolved oxygen, and ionic strength on the survival of Escherichia coli O157:H7 in organic acid solutions

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 dissolved oxygen conditions (≤0.05 and 5 mg/liter) and a range of ionic strengths (0.086 to 1.14). All solutions contained 20 mM gluconic acid, which was used as a noninhibitory 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/liter dissolved oxygen (present in tap water) over a range of ionic strengths at pH 3.2 exhibited a decrease in survival over 6 h at 30°C as the ionic strength was increased. Cells challenged in 40 mM protonated L-lactic and acetic acid solutions with ionic strength of 0.684 achieved a >4.7-log CFU/ml reduction at pH 3.2. However, under oxygen-limiting conditions in an anaerobic chamber, with ≤0.05-mg/liter oxygen, E. coli O157:H7 cells showed ≤1.55-log CFU/ml reduction regardless of pH, acid type, concentration, or ionic strength. 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.