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Antimicrob Agents Chemother. 1976 December; 10(6): 869-871
Copyright © 1976 American Society for Microbiology. All Rights Reserved.

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

¹ Departments of Pediatrics (Infectious Diseases) and Microbiology, McGill University-Montreal Children's Hospital Research Institute, Montreal, Quebec H3H 1P3, Canada

ABSTRACT

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 10⁶ colony-forming units (CFU)/ml. The clearest and most reproducible zone diameters were obtained with a 10⁷-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 <20 mm were resistant to sulfadiazine, whereas those with zone diameters of 30 mm were susceptible. False susceptible and false resistant readings were obtained with 300-µg sulfisoxazole disks. These data suggest that inocula and type of sulfonamide are critical factors in the disk diffusion test for meningococcal susceptibility testing. Sulfonamide disks are not interchangeable for susceptibility testing of meningococci.