Phenotypic characterization of R-factor tetracycline resistance determinants

SUMMARY The tetracycline-resistance determinants of R-factors from different compatibility groups have been tested in Escherichia coli K12 and phenotypically classified into two major classes. Class I determinants confer high-level resistance to tetracycline (> 100 μg/ml) and moderate resistance to minocycline (5–25 μg/ml) while those of Class II gave moderate resistance to tetracycline (50–70 μg/ml) and low resistance to minocycline. Each class was subdivided because of variation in resistance profiles and in the abilities of tetracycline and minocycline to induce increased resistance. Strains carrying two compatible Tet r R-factors of the same or different phenotypic groups did not show increased tetracycline resistance.