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Applied and Environmental Microbiology, June 2009, p. 3605-3610, Vol. 75, No. 11
0099-2240/09/$08.00+0     doi:10.1128/AEM.02462-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Escherichia coli and Salmonella enterica Are Protected against Acetic Acid, but Not Hydrochloric Acid, by Hypertonicity{triangledown}

B. Chapman1* and T. Ross2

Food Science Australia, P.O. Box 52, North Ryde, New South Wales 1670, Australia,1 Food Safety Centre, Tasmanian Institute of Agricultural Research, University of Tasmania, Private Bag 54, Hobart, Tasmania 7001, Australia2

Received 27 October 2008/ Accepted 29 March 2009

Chapman et al. (B. Chapman, N. Jensen, T Ross, and M. B. Cole, Appl. Environ. Microbiol. 72:5165-5172, 2006) demonstrated that an increased NaCl concentration prolongs survival of Escherichia coli O157 SERL 2 in a broth model simulating the aqueous phase of a food dressing or sauce containing acetic acid. We examined the responses of five other E. coli strains and four Salmonella enterica strains to increasing concentrations of NaCl under conditions of lethal acidity and observed that the average "lag" time prior to inactivation decreases in the presence of hydrochloric acid but not in the presence of acetic acid. For E. coli in the presence of acetic acid, the lag time increased with increasing NaCl concentrations up to 2 to 4% at pH 4.0, up to 4 to 6% at pH 3.8, and up to 4 to 7% (wt/wt of water) NaCl at pH 3.6. Salmonella was inactivated more rapidly by combined acetic acid and NaCl stresses than E. coli, but increasing NaCl concentrations still decreased the lag time prior to inactivation in the presence of acetic acid; at pH 4.0 up to 1 to 4% NaCl was protective, and at pH 3.8 up to 1 to 2% NaCl delayed the onset of inactivation. Sublethal injury kinetics suggest that this complex response is a balance between the lethal effects of acetic acid, against which NaCl is apparently protective, and the lethal effects of the NaCl itself. Compared against 3% NaCl, 10% (wt/wt of water) sucrose with 0.5% NaCl (which has similar osmotic potential) was found to be equally protective against adverse acetic acid conditions. We propose that hypertonicity may directly affect the rate of diffusion of acetic acid into cells and hence cell survival.

* Corresponding author. Mailing address: Food Science Australia, P.O. Box 52, North Ryde, NSW 1670, Australia. Phone: (612) 9490-8470. Fax: (612) 9490-8499. E-mail: belinda.chapman{at}csiro.au

{triangledown} Published ahead of print on 3 April 2009.

Applied and Environmental Microbiology, June 2009, p. 3605-3610, Vol. 75, No. 11
0099-2240/09/$08.00+0     doi:10.1128/AEM.02462-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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