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Applied and Environmental Microbiology, May 2005, p. 2493-2500, Vol. 71, No. 5
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.5.2493-2500.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Variation in Acid Resistance among Shiga Toxin-Producing Clones of Pathogenic Escherichia coli{dagger}

Teresa M. Large, Seth T. Walk, and Thomas S. Whittam*

Microbial Evolution Laboratory, National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824

Received 27 July 2004/ Accepted 22 November 2004

Pathogenic strains of Escherichia coli, such as E. coli O157:H7, have a low infectious dose and an ability to survive in acidic foods. These bacteria have evolved at least three distinct mechanisms of acid resistance (AR), including two amino acid decarboxylase-dependent systems (arginine and glutamate) and a glucose catabolite-repressed system. We quantified the survival rates for each AR mechanism separately in clinical isolates representing three groups of Shiga toxin-producing E. coli (STEC) clones (O157:H7, O26:H11/O111:H8, and O121:H19) and six commensal strains from ECOR group A. Members of the STEC clones were not significantly more acid resistant than the commensal strains when analyzed using any individual AR mechanism. The glutamate system provided the best protection in a highly acidic environment for all groups of isolates (<0.1 log reduction in CFU/ml per hour at pH 2.0). Under these conditions, there was notable variation in survival rates among the 30 O157:H7 strains, which depended in part on Mg2+ concentration. The arginine system provided better protection at pH 2.5, with a range of 0.03 to 0.41 log reduction per hour, compared to the oxidative system, with a range of 0.13 to 0.64 log reduction per hour. The average survival rate for the O157:H7 clonal group was significantly less than that of the other STEC clones in the glutamate and arginine systems and significantly less than that of the O26/O111 clone in the oxidative system, indicating that this clonal group is not exceptionally acid resistant with these specific mechanisms.

* Corresponding author. Mailing address: NFSTC, 165 Food Safety & Toxicology Building, Michigan State University, East Lansing, MI 48824. Phone: (517) 432-3100, ext. 178. Fax: (517) 432-2310. E-mail: whittam{at}msu.edu.

{dagger} Supplemental material for this article may be found at http://aem.asm.org/.

Applied and Environmental Microbiology, May 2005, p. 2493-2500, Vol. 71, No. 5
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.5.2493-2500.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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