Reevaluation of the Disk Diffusion Method for Sulfonamide Susceptibility Testing of Neisseria meningitidis

Lack of correlation between quantitative minimal inhibitory concentration (MIC) determinations and disk diffusion susceptibility tests in our laboratory prompted a study to reevaluate the use of the disk diffusion test for sulfonamide susceptibility testing of Neisseria meningitidis . One hundred and sixty-three recent clinical isolates of N. meningitidis were examined for sulfonamide susceptibility by the agar dilution and disk diffusion methods. Optimal inocula for each of the tests were determined, and thereafter all disk diffusion tests were compared with quantitative MICs as determined by the agar dilution method using sulfadiazine and an inoculum of 10 6 colony-forming units (CFU)/ml. The clearest and most reproducible zone diameters were obtained with a 10 7 -CFU/ml inoculum in the disk diffusion test. There was complete correlation between the disk zone diameters for 300-μg disks of sulfadiazine and sulfathiazole and the agar dilution test MICs. All isolates with a zone diameter of