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Characterization of Enterohemorrhagic Escherichia coli Strains Based on Acid Resistance Phenotypes

Arvind A. Bhagwat,^1* Lynn Chan,^1, Rachel Han,^1, Jasmine Tan,^1, Mahendra Kothary,² Junia Jean-Gilles,² and Ben D. Tall²

Produce Quality and Safety Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Agricultural Research Service, USDA, Bldg. 002, 10300 Baltimore Avenue, Beltsville, Maryland 20705-2350,¹ Division of Virulence Assessment, Food and Drug Administration, Laurel, Maryland 20708²

Received 6 December 2004/ Returned for modification 27 January 2005/ Accepted 9 March 2005

Acid resistance is perceived to be an important property of enterohemorrhagic Escherichia coli strains, enabling the organisms to survive passage through the acidic environment of the stomach so that they may colonize the mammalian gastrointestinal tract and cause disease. Accordingly, the organism has developed at least three genetically and physiologically distinct acid resistance systems which provide different levels of protection. The glutamate-dependent acid resistance (GDAR) system utilizes extracellular glutamate to protect cells during extreme acid challenges and is believed to provide the highest protection from stomach acidity. In this study, the GDAR system of 82 pathogenic E. coli isolates from 34 countries and 23 states within the United States was examined. Twenty-nine isolates were found to be defective in inducing GDAR under aerobic growth conditions, while five other isolates were defective in GDAR under aerobic, as well as fermentative, growth conditions. We introduced rpoS on a low-copy-number plasmid into 26 isolates and were able to restore GDAR in 20 acid-sensitive isolates under aerobic growth conditions. Four isolates were found to be defective in the newly discovered LuxR-like regulator GadE (formerly YhiE). Defects in other isolates could be due to a mutation(s) in a gene(s) with an as yet undefined role in acid resistance since GadE and/or RpoS could not restore acid resistance. These results show that in addition to mutant alleles of rpoS, mutations in gadE exist in natural populations of pathogenic E. coli. Such mutations most likely alter the infectivity of individual isolates and may play a significant role in determining the infective dose of enterohemorrhagic E. coli.

Dedicated to Irving Newman, who passed away on 5 February 2005.

Editor: A. D. O'Brien

Permanent address: School of Life Sciences and Chemical Technology, Ngee Ann Polytechnic, 535 Clement Road, Singapore 599489, Singapore.

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