In vitro susceptibility of Neisseria gonorrhoeae to nine antimicrobial agents.

The in vitro action of nine antibiotics was tested by the agar streak method against 45 gonococcal strains isolated from penicillin-therapy failures. The penicillin susceptibility range of these strains was 0.003 to 1.32 mug/ml, and the tetracycline susceptibility range was 0.125 to 2.0 mug/ml. Minimal inhibitory concentrations of minocycline and doxycycline paralleled the activity of tetracycline and ranged from 0.125 to 1.0 mug/ml and 0.125 to 2.0 mug/ml, respectively. Rifampicin, with a narrow range of 0.5 to 1.0 mug/ml, inhibited 75% of the strains at 0.5 mug/ml. The range for cephaloridine and cephaloglycine was 0.5 to 20.0 mug/ml, but another cephalosporium derivative, cephalexin, exhibited greater activity in its range of 0.25 to 20.0 mug/ml. A semisynthetic penicillin, carbenicillin, with a range of 0.025 to 0.75 mug/ml, displayed more activity against the lower susceptible penicillin G gonococcal strains.