Submitted to: Interscience Conference on Antimicrobial Agents & Chemotherapy Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 9/22/2001
Publication Date: N/A
Technical Abstract: Chloramphenicol is a broad-spectrum antibiotic that has been banned from use in food animals since the mid-1980s. A survey of Escherichia coli isolated in 1998 from neonatal swine with diarrhea found that 47 of 90 isolates (52%) carried the cmlA gene, which encodes a putative drug- efflux pump conferring resistance to chloramphenicol. The cmlA genotype was not associated with any specific serogroup or ribogroup, suggesting that the prevalence of chloramphenicol resistance may be due to wide dissemination of a cmlA containing plasmid. Southern blot analysis of plasmid DNA detected at least 3 distinct plasmid clones of 94, 145, and 245 kilobase-pairs (kbp), that possess the cmlA gene. The mobility of cmlA was demonstrated by conjugation of chloramphenicol resistance between isolates containing each plasmid type and an E. coli JM109 recipient strain. The transconjugants were assayed for susceptibility to the following 6 antimicrobials to which the donor strains were resistant: ampicillin (A), chloramphenicol (C), streptomycin (S), sulfamethoxazole (Su), tetracycline (T), and kanamycin (K). Transconjugants derived from donors with either the 94 kbp, 145 kbp, or 245 kbp plasmid each displayed a distinct resistance phenotype: CSuT, ACSuK, and CSSuK, respectively. Our data shows that the prevalence of chloramphenicol resistance in swine E. coli is due to dissemination of multiple plasmids that possess the cmlA gene. We have also shown that cmlA is phenotypically linked with resistance to antimicrobials that are currently approved for use in food animals, which may be a contributing factor to persistence of chloramphenicol resistance.