SUMMARY: Eighty-one strains of that appeared to be tetracycline resistant on the basis of a preliminary disc-diffusion test were examined for resistance to tetracycline and to the semi-synthetic tetracycline, minocycline. Minimum inhibitory concentration (m.i.c.) values for both drugs were determined after induction of the strains by growth for 2 h in sub-inhibitory concentrations of tetracycline. Forty-seven strains (58%) had m.i.c. values for minocycline of 12.5 μg/ml or greater, and were considered to be minocycline resistant. An additional ten strains had m.i.c. values for minocycline of 3.125 to 6.25 μg/ml and were classified as low-level resistant strains. It appears, therefore, that a fairly high proportion of tetracycline-resistant strains isolated at the present time are resistant to concentrations of minocycline unattainable with the recommended dosage for this antibiotic (Frisk & Tunevall, 1969).

Transductional analysis of the genetic determinants for tetracycline resistance revealed the existence of two types of resistance to high concentrations of tetracycline. Strains in the first category (A) were inducibly resistant to tetracycline but sensitive to minocycline; in these strains the resistance determinant was plasmid-borne. Strains in the second category (B) were resistant to both tetracycline and minocycline and had low induction ratios for tetracycline resistance; the genetic determinant for resistance in these strains was chromosomal. In addition, certain strains in category A were found to carry a chromosomal gene controlling low-level resistance to tetracycline and minocycline. This low-level resistance to tetracycline was masked in the presence of the tetracycline plasmid but could be demonstrated after loss of the plasmid. The results suggest that more than one mechanism of resistance to tetracyclines may exist in staphylococci.


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