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Applied and Environmental Microbiology, January 2006, p. 660-664, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.660-664.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Protective Effects of Organic Acids on Survival of Escherichia coli O157:H7 in Acidic Environments{dagger}

K. Bjornsdottir, F. Breidt Jr.,* and R. F. McFeeters

U.S. Department of Agriculture, Agricultural Research Service, and North Carolina Agricultural Research Service, North Carolina State University, Department of Food Science, Raleigh, North Carolina 27695-7624

Received 25 January 2005/ Accepted 1 October 2005

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 noninhibitory low-pH buffer, we investigated the killing of Escherichia coli O157:H7 strains in the presence or absence of selected organic acids (pH of 3.2), with ionic strength adjusted to 0.60 to 0.68. During a 6-h exposure period in buffered solution (pH 3.2), we found that a population of acid-adapted E. coli O157:H7 strains was reduced by 4 log cycles in the absence of added organic acids. Surprisingly, reduced lethality for E. coli O157:H7 was observed when low concentrations (5 mM) of fully protonated acetic, malic, or L-lactic acid were added. Only a 2- to 3-log reduction in cell counts was observed, instead of the 4-log reduction attributed to pH effects in the buffered solution. Higher concentrations of these acids at the same pH aided in the killing of the E. coli cells, resulting in a 6-log or greater reduction in cell numbers. No protective effect was observed when citric acid was added to the E. coli cells. D-Lactic acid had a greater protective effect than other acids at concentrations of 1 to 20 mM. Less than a 1-log decrease in cell numbers occurred during the 6-h exposure to pH 3.2. To our knowledge, this is the first report of the protective effect of organic acids on the survival of E. coli O15:H7 under low-pH conditions.

* Corresponding author. Mailing address: North Carolina State University, Department of Food Science, Raleigh, NC 27695-7624. Phone: (919) 513-0186. Fax: (919) 513-0180. E-mail: breidt{at}ncsu.edu

{dagger} Paper no. FSR04-37 of the Journal Series of the Department of Food Science, NC State University, Raleigh, NC 27695-7624.

Applied and Environmental Microbiology, January 2006, p. 660-664, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.660-664.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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