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U.S. Department of Health and Human Services

Animal & Veterinary

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FDA Veterinarian Newsletter September/October 2001 Volume XVI, No V


We have characterized the antimicrobial susceptibility patterns of 91 isolates of Escherichia coli associated with diarrhea in neonatal pigs from multiple farms in Oklahoma. Minimum inhibitory concentrations were determined for 17 antimicrobials that are monitored by the National Antimicrobial Resistance Monitoring System. Based on resistance breakpoints determined by the National Committee for Clinical Laboratory Standards, 88 of 91 isolates (97%) were resistant to at least one antibiotic and 81 of 91 (89%) were resistant to four or more antibiotics.

The broad-spectrum antibiotic chloramphenicol (CML) has been removed from use in food animals since 1985, yet we observed CML resistance in 47/91 (52%) of these isolates. The cmlA gene, which encodes a CML efflux pump, was detected by polymerase chain reaction in 46 of the CML resistant isolates, and 4 of these also possessed the cat2 gene encoding a chloramphenicol acetyltransferase. The one CML resistant isolate that did not contain either cmlA or cat2 did, however, possess the flo gene, which encodes an efflux pump that confers resistance to both florfenicol and CML. The genetic relatedness of all 91 isolates was assayed by ribotyping. Seventeen distinct ribogroups were identified but 72% of the isolates clustered into 6 major ribogroups. CML resistance was found in all but one of the major ribogroups, the largest containing 31 isolates with 23/31 resistant to CML. Our data suggests that the cmlA resistance genotype is widely disseminated in enterotoxigenic E. coli isolated from swine, and that the chloramphenicol resistance phenotype persists even in the absence of CML selection pressure.