Aerobic and anaerobic susceptibility tests with three tetracyclines. Reassessment of the "class concept" of disk testing.

Both aerobic and anaerobic susceptibility tests were performed with tetracycline, doxycycline, and minocycline, using disk diffusion and agar dilution technics. The data were examined in order to reassess the concept of testing a single disk, representative of the tetracycline class of antimicrobics. All strains that were susceptible to a tetracycline disk were predictably susceptible to the tetracycline analogs. Some strains that gave zones in the resistant or intermediate range were susceptible or moderately susceptible to doxycycline and minocycline by agar dilution methods. However, disk tests with the more active analogs were often unproductive, since most tetracycline-resistant strains gave indeterminate results with doxycycline or minocycline disks. It was concluded that the "class concept" of disk testing is still appropriate and that tests with tetracycline disks predict susceptibility to the other tetracycline analogs reasonably well.