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Applied and Environmental Microbiology, January 2004, p. 1-7, Vol. 70, No. 1
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.1.1-7.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Characterization of Multiple-Antimicrobial-Resistant Salmonella Serovars Isolated from Retail Meats

Sheng Chen,1 Shaohua Zhao,2 David G. White,2 Carl M. Schroeder,1,{dagger} Ran Lu,3 Hanchun Yang,4 Patrick F. McDermott,2 Sherry Ayers,2 and Jianghong Meng1*

Department of Nutrition and Food Science, University of Maryland, College Park, Maryland,1 Division of Animal and Food Microbiology, Office of Research, Center for Veterinary Medicine, U.S. Food & Drug Administration, Laurel, Maryland,2 Institute for Food Safety & Inspection, Ministry of Health,3 China Agricultural University, Beijing, People's Republic of China4

Received 16 April 2003/ Accepted 10 September 2003

A total of 133 Salmonella isolates recovered from retail meats purchased in the United States and the People's Republic of China were assayed for antimicrobial susceptibility, the presence of integrons and antimicrobial resistance genes, and horizontal transfer of characterized antimicrobial resistance determinants via conjugation. Seventy-three (82%) of these Salmonella isolates were resistant to at least one antimicrobial agent. Resistance to the following antibiotics was common among the United States isolates: tetracycline (68% of the isolates were resistant), streptomycin (61%), sulfamethoxazole (42%), and ampicillin (29%). Eight Salmonella isolates (6%) were resistant to ceftriaxone. Fourteen isolates (11%) from the People's Republic of China were resistant to nalidixic acid and displayed decreased susceptibility to ciprofloxacin. A total of 19 different antimicrobial resistance genes were identified in 30 multidrug-resistant Salmonella isolates. The blaCMY-2 gene, encoding a class A AmpC ß-lactamase, was detected in all 10 Salmonella isolates resistant to extended-spectrum ß-lactams. Resistance to ampicillin was most often associated with a TEM-1 family ß-lactamase gene. Six aminoglycoside resistance genes, aadA1, aadA2, aacC2, Kn, aph(3)-IIa, and aac(3)-IVa, were commonly present in the Salmonella isolates. Sixteen (54%) of 30 Salmonella isolates tested had integrons ranging in size from 0.75 to 2.7 kb. Conjugation studies demonstrated that there was plasmid-mediated transfer of genes encoding CMY-2 and TEM-1-like ß-lactamases. These data indicate that Salmonella isolates recovered from retail raw meats are commonly resistant to multiple antimicrobials, including those used for treating salmonellosis, such as ceftriaxone. Genes conferring antimicrobial resistance in Salmonella are often carried on integrons and plasmids and could be transmitted through conjugation. These mobile DNA elements have likely played an important role in transmission and dissemination of antimicrobial resistance determinants among Salmonella strains.

* Corresponding author. Mailing address: Department of Nutrition and Food Science, 0112 Skinner Building, University of Maryland, College Park, MD 20742. Phone: (301) 405-1399. Fax: (301) 314-3313. E-mail: jm332{at}umail.umd.edu.

{dagger} Present address: Food Safety and Inspection Service, United States Department of Agriculture, Washington, DC 20250-3700.

Applied and Environmental Microbiology, January 2004, p. 1-7, Vol. 70, No. 1
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.1.1-7.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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