Comparative Study of Gonococcal Susceptibility to Penicillin in the United States, 1955–1969

During the first 10 years after the introduction of penicillin therapy for gonorrhea, in-vitro susceptibility testing of 771 gonococcal isolates in the United States revealed that only 5 cultures (0.6%) required more than 0.05 unit/ml of penicillin to inhibit the organism. Susceptibility testing of 1,124 cultures isolated in 1965 from routine clinic admissions in 7 cities indicated that during the second 10-year interval there was an increase from 0.6% to 42.0% in cultures requiring more than 0.05 unit/ml of penicillin to inhibit growth. Five percent of the cultures tested required more than 0.5 unit/ml for complete inhibition. The most recent testing of susceptibility (1968-1969) of 649 cultures isolated from a comparable population in 5 cities demonstrates a continued increase in gonococcal resistance to penicillin. Since 1965, the percentage of isolates requiring more than 0.05 unit/ml to inhibit growth has increased to 65%, and the cultures requiring 0.5 unit/ml have almost tripled. Studies of 108 gonococcal isolates from patients in whom penicillin treatment failed since 1965 have shown an even greater increase in resistance.