DETERMINATIONS OF THE SENSITIVITY OF NEISSERIA GONORRHOEAE TO PENICILLIN PERFORMED ON SOLID MEDIA OF DIFFERENT COMPOSITION.

The increasing frequency of strains of Neisseria gonorrhoeae with reduced sensitivity to penicillin is making it more and more important to determine the sensitivity of individual strains before and after treatment with this antibiotic. It was recently demonstrated that the concentrations of penicillin inhibitory to Neisseria gonorrhoea depend upon the medium employed. This important observation made it desirable to have available for interlaboratory comparisons a non-commercial reproducible medium which would support the growth of the vast majority of the gonococcal strains in circulation. Using a plate dilution procedure, various modifications of the HYL medium described elsewhere were compared with the routine medium for carrying out sensitivity determinations. Both stock and fresh strains of Neisseria gonorrhoeae were used in the experiments. The 50% inhibitory concentrations observed on the two media did not differ to any great extent, being only about 40% higher on the routine medium than on the HYL medium. The difference between the HYL medium and the routine medium was somewhat greater for the less sensitive strains than for the more sensitive strains. The variations in the difference from strain to strain were larger for the more sensitive fresh strains than for the less sensitive strains and the stock strains. This observation limits the value of a correction method employing, for example, three reference strains as a means of ensuring comparability between the results of different laboratories.