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

item Breidt, Frederick

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2008
Publication Date: 12/1/2008
Citation: Kreske, A.C., Bjornsdottir, K., Breidt, F., Hassan, H. 2008. Effects of pH, dissolved oxygen, and ionic strength on the survival of Escherichia coli O157:H7 in organic acid solutions. Journal of Food Protection. 71(12):2404-2409.

Interpretive Summary: Acid resistant disease causing bacterium may be a threat to acidified foods. While most bacteria can’t grow in acidified foods, some acid resistant disease causing bacteria, including Escherichia coli O157:H7 (E. coli) can survive for extended periods surviving for weeks or even months, depending on the acid conditions, before succumbing to the killing effects of acid preservatives like vinegar. Most previous experiments to determine acid resistance of bacteria are carried out using bench top solutions that contain dissolved oxygen. We have found that dissolved oxygen can accelerate acid killing of bacteria. This report shows that E. coli may survive better in acid than expected from previously published research results, because pickled vegetables and other acidified foods are typically packaged in sealed containers that lack air. To accurately determine how long E. coli and other acid resistant disease causing bacteria can survive in acidified foods, the oxygen effect needs to be taken into account. Results from this research may be used to help assure the safe production of acidified food products, and increase the current scientific understanding of how bacteria are killed by acid solutions.

Technical 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 (less than or equal to 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 less than or equal to 0.05-mg/liter oxygen, E. coli O157:H7 cells showed less than or equal to 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.