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Journal of Clinical Microbiology, February 2002, p. 389-394, Vol. 40, No. 2
0095-1137/01/$04.00+0     DOI: 10.1128/JCM.40.2.389-394.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Characterization of Chloramphenicol Resistance in Beta-Hemolytic Escherichia coli Associated with Diarrhea in Neonatal Swine

Kenneth M. Bischoff,1* David G. White,2 Patrick F. McDermott,2 Shaohua Zhao,2 Stuart Gaines,2 John J. Maurer,3 and David J. Nisbet1

Southern Plains Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, College Station, Texas,1 Office of Research, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, Maryland,2 Department of Avian Medicine, University of Georgia, Athens, Georgia3

Received 27 August 2001/ Returned for modification 4 November 2001/ Accepted 20 November 2001

Ninety beta-hemolytic Escherichia coli isolates associated with diarrhea in neonatal pigs from multiple farms in Oklahoma were investigated for known associated disease serotypes, virulence factors, ribotypes, and antimicrobial susceptibility phenotypes. Fifteen different serotypes were observed, with 58% of isolates belonging to groups that produce one of three major enterotoxins: O149, O147, and O139. Thirty percent of the swine E. coli isolates possessed a combination of F4 fimbriae and the heat-labile toxin and heat-stable toxin B enterotoxins. Seventy-three percent of the E. coli isolates were resistant to five or more antibiotics. Interestingly, 53% of swine E. coli isolates exhibited resistance to chloramphenicol (CHL), an antibiotic whose use in food animals has been prohibited in the United States since the mid-1980s. The cmlA gene, which encodes a putative CHL efflux pump, was detected by PCR in 47 of the 48 CHL-resistant isolates, and 4 of these also possessed the cat2 gene, which encodes a chloramphenicol acetyltransferase. The one CHL-resistant isolate that did not contain either cmlA or cat-2 possessed the flo gene, which confers resistance to both florfenicol and CHL. To determine whether CHL-resistant swine E. coli isolates represented dissemination of a clonal strain, all 90 isolates were analyzed by ribotyping. Seventeen distinct E. coli ribogroups were identified, with CHL resistance observed among the isolates in all except one of the major ribogroups. The identification of the cmlA gene among diverse hemolytic enterotoxigenic E. coli strains demonstrates its broad dissemination in the swine production environment and its persistence even in the absence of CHL selection pressure.

* Corresponding author. Mailing address: Southern Plains Agricultural Research Center, USDA/ARS, 2881 F-B Rd., College Station, TX 77845. Phone: (979) 260-9306. Fax: (979) 260-9332. E-mail: bischoff{at}ffsru.tamu.edu.

Journal of Clinical Microbiology, February 2002, p. 389-394, Vol. 40, No. 2
0095-1137/01/$04.00+0     DOI: 10.1128/JCM.40.2.389-394.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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