Changing patterns of plasmid-mediated drug resistance during tetracycline therapy.

The patterns of drug resistance and the frequency of conjugative R plasmids in intestinal Escherichia coli from 88 patients treated for a skin disease (acne vulgaris) with low oral doses of tetracycline are reported. The proportion of patients with resistant bacteria was progressively greater in patients who received tetracycline for 1 week, 4 weeks, or longer (from 50 to 88%). No multiply drug-resistant bacteria were detected before treatment or after 1 week of treatment. After more than 4 weeks of treatment, multiply drug-resistant E. coli were isolated from about 50% of the patients. The origin and selection of R plasmid-determined multiple drug resistance are discussed.