Submitted to: Federation of European Microbiological Societies Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2004
Publication Date: 2/1/2005
Citation: Bischoff, K.M., White, D.G., Hume, M.E., Poole, T.L., Nisbet, D.J. 2005. The chloramphenicol resistance gene cmlA is disseminated on transferable plasmids that confer multiple-drug resistance in swine Escherichia coli. FEMS Microbiology Letters. 243:285-291. Interpretive Summary: Treating food animals with antibiotic drugs may coincidentally promote disease-causing bacteria like E. coli to acquire genes that make the bacteria resistant to antibiotics. Sometimes the bacteria will lose the resistance gene when the antibiotic is no longer used. Chloramphenicol is an antibiotic that has been prohibited from use in food animals since the mid-1980s, but we recently found that over 50% of E. coli from sick pigs carried chloramphenicol resistance genes. In the present study, we found that the chloramphenicol resistance genes are connected to other genes which encode resistance to antibiotics that are frequently used to treat sick pigs. This suggests that the use of other antibiotics could still select for chloramphenicol resistance genes even though chloramphenicol is not used in pigs. These results exemplify the difficulties in managing increased rates of antimicrobial resistance by restricting the drug's use.
Technical Abstract: A recent study of beta-hemolytic Escherichia coli isolated from diarrheic swine found that 53% were resistant to chloramphenicol (CHL), a drug that has been prohibited from use in food animals since the mid-1980s. To identify the factors governing the persistence of CHL resistance in the absence of specific selection pressure, the location of the CHL resistance gene cmlA and its linkage to other resistance determinants were investigated. Mobility of CHL resistance and linkage to other resistance genes were assessed in bacterial conjugation experiments. Plasmid DNA from 46 CHL resistant E. coli isolates and 24 transconjugant strains was characterized for the presence of integrons, drug resistance and virulence genes by Southern Blot and PCR analyses. The CHL resistance gene cmlA in swine E. coli was present on large plasmids that ranged in size from 130 to 270 kbp. Fifty-two percent of the isolates were able to transfer CHL resistance at conjugation frequencies ranging from 10**-3 to 10**-8. Antimicrobial susceptibility testing demonstrated a high level of significance for co-transfer of resistance to sulfamethoxazole, tetracycline, and kanamycin. Plasmid analysis indicated the presence of at least two distinct class 1 integrons linking cmlA to the aminoglycoside resistance genes SAT and aadA2. These results suggest that in the absence of specific selection pressure, linkage of genes encoding resistance to antimicrobials that are currently approved for use in food animals is a contributing factor to persistence of chloramphenicol resistance.