Multiple antibiotic resistance in Neisseria gonorrhoeae.

The sensitivities of gonococcal isolates to six antibiotics were determined for gonococci isolated in Philadelphia in 1972. The degree of association between susceptibilities to any two antibiotics was determined (coefficient of correlation). The correlation between penicillin and tetracycline (r = 0.75) was almost as good as that between two penicillins, penicillin G and ampicillin (r = 0.85), but the difference was statistically significant. The lowest correlation found was between erythromycin and chloramphenicol (r = 0.62), two antibiotics seldom used in gonorrhea therapy. In addition, gonococci most resistant to one antibiotic were the most likely to be multiply resistant. This was found with respect to penicillin, tetracycline, erythromycin, and chloramphenicol. Approximately 40% of gonococci classified as "most resistant" (exceeding the resistance of 75% of all isolates) to one antibiotic were also "most resistant" to three others. Finally, multiply resistant mutants were isolated by selection for resistance to either penicillin or tetracycline. These results provide evidence for the existence of a common mechanism for multiple antibiotic resistance in the gonococcus.