The effect of media on antimicrobial susceptibility testing of Neisseria gonorrhoeae

We have investigated the effect of four different media on the results of susceptibility tests for clinical and reference strains of Neisseria gonorrhoeae. Diagnostic Sensitivity Test (DST) agar supplemented with 5% lysed horse blood and 1% IsoVitaleX was regarded as the 'standard' and three other media were compared with it--DST plus 5% lysed blood, GC agar supplemented with 1% IsoVitaleX and Proteose agar plus 1% haemoglobin and 1% IsoVitaleX. We found no significant differences between the two DST media when testing penicillin, cefuroxime, tetracycline or erythromycin. Proteose agar compared well with the 'standard' for all antibiotics except erythromycin for which MICs were significantly increased. GC agar showed the poorest agreement with the 'standard' medium. Gonococci appeared more sensitive to tetracycline and less sensitive to erythromycin on this medium. The use of GC agar did not significantly affect susceptibilities to the beta-lactam agents tested, although MICs measured on GC agar did tend to be higher than on the 'standard' DST medium.