AEM Accepts, published online ahead of print on 23 October 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.01158-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Beta-Lactam Resistance in Salmonella Isolated from Retail Meats in the United States: National Antimicrobial Resistance Monitoring System (NARMS): 2002-2006

S. Zhao^*, K. Blickenstaff, A. Glenn, S. L. Ayers, S. L. Friedman, J. W. Abbott, and P. F. McDermott

Division of Animal and Food Microbiology, Office of Research, Center for Veterinary Medicine, U. S. Food & Drug Administration, Laurel, MD

^* To whom correspondence should be addressed. Email: shaohua.zhao{at}fda.hhs.gov.

Ampicillin-resistant (AMP^R) Salmonella (n=344) representing 32 serotypes isolated from retail meats from 2002-2006 were tested for susceptibility to 21 other antimicrobial agents, and screened for the presence of five beta-lactamase gene families (bla_CMY, bla_TEM, bla_SHV, bla_OXA, bla_CTX-M) and class 1 integrons. Among the AMP^R isolates, 66.9% were resistant to 5 antimicrobials and 4.9% to 10 antimicrobials. Co-resistance to other beta-lactams was noted for amoxicillin/clavulanic acid (55.5%), ceftiofur (50%), cefoxitin (50%), and ceftazidime (24.7%), whereas less than 5% of isolates were resistant to piperacillin/tazobactam (4.9%), cefotaxime (3.5%), ceftriaxone (2%), and aztreonam (1.2%). All isolates were susceptible to cefepime, imipemen and cefquinome. No Salmonella producing extended-spectrum beta-lactamases (ESBL) was found in this study. Approximately 7% of the isolates displayed a typical MDR-AmpC phenotype, resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, tetracycline (ACSSuT), plus resistance to amoxicillin/clavulanic acid, cefoxitin, and ceftiofur with decreased susceptibility to ceftriaxone (MIC 4ug/ml). PFGE results showed that several MDR clones were geographically dispersed in different types of meats throughout the five sampling years. Additionally, 50% of the isolates contained bla_CMY, 47% carried bla_TEM-1 and 2.6% carried both genes. Only 15% of the isolates contained class I integrons carrying various combinations of aadA, aadB and dfrA gene cassettes. The bla_CMY, bla_TEM and class 1 integrons were transferable through conjugation and/or transformation. Our findings indicate a varied spectrum of co-resistance traits is present in AMP^R Salmonella in the U.S. meat supply, with a continued predominance of bla_CMYand bla_TEM genes in beta-lactam resistant isolates.