Comparative Susceptibility of Penicillinase-Positive and -Negative Neisseria gonorrhoeae to 30 Antibiotics

Abstract

The minimal inhibitory concentrations of 30 antibiotics were determined by the agar dilution method for 17 penicillinase-positive and 50 penicillinase-negative strains of Neisseria gonorrhoeae . The latter included 42 strains that were penicillin susceptible (pen S) (minimal inhibitory concentration, <0.125 μg/ml) and 8 strains with intermediate resistance to penicillin (pen I; minimal inhibitory concentration, 0.125 to 0.5 μg/ml). Two penicillinase-resistant penicillins (methicillin and nafcillin) were inhibitory for penicillin-resistant (pen R) strains. Three new cephalosporins (cefuroxime, cefamandole, cefaclor) and a cephamycin (cefoxitin) were bacteriostatic (minimal inhibitory concentration ≤0.8 μg/ml) for 90% of pen S, pen I, and pen R strains. Pen I strains were more resistant than pen R strains to 6 of 13 cephalosporins. Rifampin, erythromycin, spectinomycin, chloramphenicol, and the tetracyclines were inhibitory for both pen S and pen R strains. The minimal bactericidal concentrations of cefuroxime, cefamandole, cefaclor, and cefoxitin were measured for 17 pen R strains and eight pen I strains by serial dilution of the antibiotics in Trypticase soy broth supplemented with 1% Iso VitaleX and 1% hemoglobin. All tubes were subcultured after overnight incubation at 37°C. Cefuroxime and cefoxitin were bactericidal at low concentrations (minimal bactericidal concentration, ≤1.0 μg/ml) for 16 of 17 pen R strains and 6 of 8 pen I strains.