AEM Accepts, published online ahead of print on 30 October 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.00210-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Multilocus variable number of tandem repeats analysis and plasmid profiling to study the occurrence of bla_CMY-2 within a PFGE-defined clade of Salmonella enterica serovar Typhimurium

B. Adhikari, T. E. Besser, J. M. Gay, L. K. Fox, D. D. Hancock, and M. A. Davis^*

AAHP, Field Disease Investigation Unit, Department of Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States; Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA 99164, United States

^* To whom correspondence should be addressed. Email: madavis{at}vetmed.wsu.edu.

Salmonella enterica serovar Typhimurium circulating in food animal populations and carrying resistance to antimicrobial agents represents a human health risk. Recently, a new clade of S. Typhimurium, WA-TYP035/187, was reported in cattle and humans in the Pacific Northwest, United States of America. The objective of this study was to describe a possible mechanism of acquisition of third generation cephalosporin resistance in this clade. Ceftazidime resistance increased steadily among WA-TYP035/187 isolates from 0% (0/2) in 1999 to77.8% (28/36) in 2006 (² for linear trend, p-value < 0.001). Among 112 bovine-source and 18 human-source isolates, 49 (43.8%) and 12 (66.7%) were resistant to ceftazidime, respectively. Multiple-locus variable number tandem repeat analysis (MLVA) and plasmid profiling suggested that resistance was acquired by multiple independent genetic events within the WA-TYP035/187 clade. Given the lack of an obvious reservoir in species other than cattle and a parallel rise in ceftiofur resistance in the bovine-specific serovar S. Dublin in the same time frame and region, selection pressure due to the use of the third generation cephalosporin drug ceftiofur in cattle is a likely factor driving the increasing cephalosporin resistance of WA-TYP035/187.