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Title: Escherichia coli O157:H7 strains isolated from environmental sources differ significantly in acid resistance compared to human outbreak strains

item Berry, Elaine
item Cooley, Michael
item Mandrell, Robert
item Breidt, Frederick

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2008
Publication Date: 3/1/2009
Citation: Oh, D., Pan, Y., Berry, E.D., Cooley, M.B., Mandrell, R.E., Breidt, F. 2009. Escherichia coli O157:H7 strains isolated from environmental sources differ significantly in acid resistance compared to human outbreak strains. Journal of Food Protection. 72(3):503-509.

Interpretive Summary: Many disease causing strains of foodborne microorganism (E. coli) were analyzed to determine if the source from which they were isolated had an effect on the resistance of the strains to acid. Acid conditions typical of food products were used in the study. The results of the strain survey showed that the original source of the bacterial strain (cows, cow feces, water, human disease isolate) was related to the acid resistance for a given strain. It was found that human isolates were less acid resistant than animal or environmental isolates for some acid conditions tested, but not others. This was somewhat surprising since acid resistance is needed for the bacteria to get through the stomach acid to cause disease. Understanding acid resistance characteristics of these disease causing bacteria may help prevent human infections.

Technical Abstract: A number of studies on the influence of acid on Escherichia coli O157:H7 have shown considerable strain differences, but limited information has been reported to compare the acid resistance based on the different sources of E. coli O157:H7 isolates. The purpose of this study was to determine the survivability of E. coli O157:H7 strains isolated from five sources (foods, bovine carcasses, bovine feces, water, and human) in 400 mM acetic acid solutions under conditions that are typical of acidified foods. The isolates from bovine carcasses, feces and water survived acetic acid treatment at pH 3.3 and 30oC significantly (P is less than or equal to 0.05) better than food or human isolates. However, resistance to acetic acid significantly increased as temperature decreased to 15°C for a given pH, with little difference (P is greater than or equal to 0.05) among the different isolation sources. All groups of E. coli O157:H7 strains showed more than 1.8 to 4.5 log reduction at pH 3.3 and 30°C after 25 min. Significantly reduced lethality (less than 1 log reduction) for all E. coli O157:H7 strain mixtures was observed when pH increased to 3.7 or 4.3, with little difference in acetic acid resistance among the groups. The addition of glutamate to the acetic acid solution or anaerobic incubation provided the best protection compared to the above conditions for all groups of isolates. These results suggest that temperature, pH and atmospheric condition are key factors in establishing strategies for improving the safety of acidified foods.