Protective effects of organic acids on survival of Escherichia coli O157:H7 in acidic environments

Authors: BJORNSDOTTIR, K - NCSU; Breidt, Frederick; McFeeters, Roger

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2005
Publication Date: 1/1/2006
Citation: Bjornsdottir K, Breidt F, McFeeters RF. 2006. Protective effects of organic acids on survival of Escherichia coli 0157:H7 in acidic environments. Appl Environ Microbio 72:660-664.

Interpretive Summary: Outbreaks of disease due to acid-tolerant bacteria in acidic foods, including apple cider and orange juice, have raised questions about the safety of acidified foods in general. A clear understanding of how different acids, commonly found in or used in foods, can kill disease-causing bacteria is lacking. By using novel methods for determining acid-killing effects on bacteria that are related to the specific type of acid used, a surprising result was found. Under some conditions, selected acids were found to confer a protective effect on an acid-resistant, disease-causing bacterium, Escherichia coli, by aiding in the survival of that organism under acid conditions that are similar to conditions found in some acidified foods. In this report, we define the acid concentrations and conditions under which these protective effects can be observed. To our knowledge this kind of protective effect has not been previously recorded in the scientific literature. Further investigation of this phenomenon may eventually lead to specific recommendations for improving the safety acidic food products.

Technical Abstract: 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 non-inhibitory low pH buffer, we investigated the destruction Escherichia coli O157:H7 strains in the presence or absence of selected organic acids at pH 3.2. During a 6-h exposure period in buffered solution at pH 3.2, we found that a population of E. coli O157:H7 strains (prepared to induce acid resistance) was reduced by 4 log cycles at pH 3.2 in the absence of additional organic acids. Surprisingly, some protective effects were observed when acetic, malic or L-lactic acids were added at low concentrations (5 mM), resulting in an increase in the survival of the E. coli O157:H7 strains, and yielding only a 2-3-log reduction in cell counts under similar conditions. Higher concentrations of these acids aided in the destruction of the E. coli cells, resulting in a 6-log or greater reduction in cell numbers. When D-lactic acid was added at concentrations of 5-20 mM, or citric acid at concentrations greater than 100 mM, strong protective effects were observed, resulting in less than a 1-log drop in cell numbers during the 6-h exposure to pH 3.2. To our knowledge, this is the first report of the protective effects of D lactic acid and other organic acids on the survival of E. coli O157:H7 under acidic conditions.