Association between the auxogroup of Neisseria gonorrhoeae and the minimal inhibitory concentration of penicillin.

The auxogroups and the minimal inhibitory concentrations (MICs) of penicillin for 885 recently isolated strains of Neisseria gonorrhoeae were determined. Auxogroups were established according to the growth requirements of the strains for proline (Pro-), uracil (Ura-), hypoxanthine (Hyx-), citrulline (Cit-), or citrulline replaceable by ornithine (Orn-). The seven most common auxogroups studied were: Pro-Cit-Ura- (38.8%), Orn-Hyx-Ura- (24.1%), nonrequiring (NR) strains (13.4%), Pro- (9.7%), Orn- (5.2%), Cit-Hyx-Ura- (4.2%), and Pro-Onr-Hyx-Ura- (2.0%). The distribution of MICs of penicillin for the complete set of 885 strains had a bimodal distribution, with a minor peak at 0.01 microgram of penicillin/ml and a major one at 0.16 microgram/ml. The auxogroups, however, had different patterns of distribution of MICs of penicillin. Five patterns of distribution were found among eight auxogroups identified. (1) Orn- and NR auxogroups showed a broad distribution of MICs, with peak at 0.08 microgram/ml. (2) Of the Cit-Hyx-Ura- and Orn-Hyx-Ura- auxogroups, greater than 98% of strains were susceptible to less than or equal to 0.04 microgram of penicillin/ml. (3) All strains of the Pro-Orn-Hyx-Ura- and Pro-Cit-Hyx-Ura- auxogroups were susceptible to less than or equal to 0.08 microgram of penicillin/ml. (4) More than 95% of strains of the Pro-Cit-Ura- auxogroup required greater than of equal to 0.08 microgram of penicillin/ml for inhibition of growth. (5) The Pro- auxogroup showed a bimodal distribution, with a major peak at 0.01 microgram of penicillin/ml and a minor one at 0.32 microgram/ml.