R plasmid with carbadox resistance from Escherichia coli of porcine origin.

Escherichia coli isolates of porcine fecal origin from a farm where the antibacterial agent carbadox was used were examined for resistance to carbadox (Cdxr). Of 72 strains examined, 24 showed resistance to this drug. All 24 Cdxr strains, except one, were also resistant to tetracycline (Tcr), streptomycin (Smr), spectinomycin (Spcr), sulfadimethoxine (Sur), kanamycin (Kmr), ampicillin (Apcr), or a combination of tetracycline, streptomycin, spectinomycin, sulfadimethoxine, and ampicillin. The Cdxr character was invariably transmissible by conjugation to E. coli K-12 jointly with other drug resistance, with the resistance patterns present in transconjugants being Cdxr Smr Spcr Apcr or Cdxr Smr Spcr Sur Apcr. About 25% of these transconjugants simultaneously lost the resistance to carbadox, streptomycin, spectinomycin, and ampicillin or carbadox, streptomycin, spectinomycin, sulfadimethoxine, and ampicillin or carbadox, streptomycin, spectinomycin, sulfadimethoxine, and ampicillin in the presence of acriflavine. Agarose gel electrophoretic analysis of deoxyribonucleic acid from a transconjugant showed a single plasmid deoxyribonucleic acid molecule with a molecular weight of about 28 x 10(6), which was capable of transforming E. coli C to Cdxr Smr Spcr Apcr. This resistance was transmissible by conjugation as a unit to other K-12 strains. These results confirmed the presence of an R plasmid specifying the Cdxr character in the host strain.