Rectal swabs were taken from healthy household dogs that, insofar as could be determined, had not received antimicrobial drugs. Tetracycline-resistant coliforms comprised 80 to 100% of the total number of coliforms in 61 (65%) of the 94 dogs sampled. The median number of other resistance determinants possessed by these tetracycline-resistant coliforms was 5.1. Of the tetracycline-resistant strains studied, 97% were resistant to streptomycin; 76% were resistant to sulfonamides; 59% were resistant to ampicillin; 59% were resistant to kanamycin/neomycin; and 40% were resistant to chloramphenicol. A total of 64% of the strains was shown to transfer resistance by conjugation or by the aid of the sex factor F. Of the strains transferring resistance, 33% were found to transfer all of their resistance determinants.
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