Activity of Minocycline Against R Factor-Carrying Enterobacteriaceae1

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Minocycline (Min) and other tetracyclines were tested in vivo and in vitro for activity against strains of salmonellae harboring R factor-mediated chlortetracycline (A) resistance. Minocycline was more active in vitro against these strains than the other tetracyclines. Mice infected with these cultures responded to treatment with Min, but there was little or no response to treatment with the other tetracyclines. Various A-resistant field isolates of Salmonella typhimurium and Escherichia coli were all shown to be more susceptible in vitro to Min than to A. Several of these cultures were shown to transfer A resistance. Cultures of normal bacterial flora were isolated from swine fecal matter after plating on Min-supplemented Mac-Conkey agar. All recovered cultures were more susceptible in vitro to Min than to A. Several isolates were able simultaneously to transfer Min and A resistances into S. choleraesuis var. kunzendorf; however, the levels of Min resistance in either the donors or exconjugants were no higher than in those cultures of salmonellae which responded in vivo to Min therapy. The same was true for the levels of Min resistance in the field isolates of S. typhimurium and E. coli. The increased potency of Min, as compared to tetracycline, against R factor-carrying bacteria was not observed for organisms made resistant (presumably chromosomally) to A by successive transfers in increasing concentrations of the agent, suggesting different mechanisms as the basis for chromosomal versus R factor-mediated A resistances.

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