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A recent study of β-hemolytic
Escherichia coli isolated from diarrheic swine found that 53% were resistant to chloramphenicol, a drug that has been prohibited from use in food animals in the US since the mid-1980s. To identify the factors governing the persistence of chloramphenicol resistance in the absence of specific selection pressure, the location of the chloramphenicol resistance gene
cmlA and its linkage to other resistance determinants were investigated. Southern blot analysis of plasmid DNA from 46 swine
E. coli isolates indicated that
cmlA was present on large plasmids greater than 100 kbp. Fifty-two percent of the isolates were able to transfer chloramphenicol resistance to an
E. coli recipient at conjugation frequencies ranging from 10
−3 to 10
−8 per recipient. Antimicrobial susceptibility tests on transconjugant strains demonstrated that resistance to sulfamethoxazole, tetracycline, and kanamycin frequently transferred along with chloramphenicol resistance. The transconjugant strains possessed at least two distinct class 1 integrons that linked
cmlA to both aminoglycoside resistance genes
aadA1 and
aadA2 and either to
sul1 or to
sul3 sulphonamide resistance genes. These results suggest that in the absence of specific chloramphenicol selection pressure, the
cmlA gene is maintained by virtue of gene linkage to genes encoding resistance to antimicrobials that are currently approved for use in food animals.