Submitted to: Journal of Clinical Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2001
Publication Date: 2/1/2002
Interpretive Summary: Disease causing (pathogenic) microorganisms can be a serious obstacle to efficient livestock production. In particular, different strains of the bacterium, Escherichia coli, can infect and kill baby pigs. The use of antibiotics to prevent losses of baby pigs caused by E. coli is an important therapy available to swine producers; however, over time the bacterium can become resistant to the antibiotics making effective disease management difficult or impossible. We characterized the antibiotic resistance profiles of 90 pathogenic E. coli isolated from baby pigs. Ninety-seven percent of the isolates were resistant to at least one antibiotic and 73% were resistant to five or more antibiotics. Interestingly, 53% of the isolates were resistant to chloramphenicol (CML), an antibiotic that has not been approved for use in food animals in the U.S. since the mid 1980s. This is important information because it shows that even though CML has not been used in agriculture for several years, resistance to it still remains. This work also shows that resistance of pathogenic E. coli strains to a number of other antibiotics may become a problem for swine producers in the future unless these antibiotics are used appropriately and judiciously.
Technical Abstract: Ninety hemolytic Escherichia coli isolates associated with diarrhea in neonatal pigs from multiple farms in Oklahoma were investigated for antimicrobial susceptibility phenotypes. Minimum inhibitory concentrations were determined for 17 antimicrobials that are monitored by the National Antimicrobial Resistance Monitoring System. Ninety-seven percent of E. coli isolates were resistant to at least one antibiotic, and 73% were resistant to five or more antibiotics. Interestingly, 48 of 90 (53%) of swine E. coli isolates exhibited resistance to chloramphenicol (CML), an antibiotic that has not been in use in food animals in the U.S. since the mid 1980s. The cmlA gene, which encodes a putative CML efflux pump, was detected by polymerase chain reaction in 47 of the 48 CML resistant isolates, and 4 of these also possessed the cat2 gene encoding a CML acetyltransferase. To determine whether CML 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 83% of the isolates clustering into 6 major ribogroups. CML resistance was observed in all of the major ribogroups with the exception of one. The identification of the cmlA gene among diverse hemolytic ETEC strains suggests broad dissemination of this genotype in the swine production environment, and that the CML resistance phenotype persists even in the absence of CML selection pressure.