STANDARDIZED DISK-DIFFUSION SUSCEPTIBILITY TEST FOR HEMOPHILUS-INFLUENZAE

The emergence of ampicillin-resistant strains of H. influenzae emphasized the need for an improved practical method for routine susceptibility testing of clinical isolates. A simplified medium for quantitative dilution susceptibility testing that is composed of Mueller-Hinton medium plus Supplement C (Difco) was previously described. In the present study, paired broth-dilution and disk-diffusion susceptibility tests with ampicillin and chloramphenicol were performed on 100 strains of Haemophilus (95 H. influenzae and 5 H. parainfluenzae), including 30 strains with previously documented ampicillin resistance. Disk-diffusion tests were performed in exactly the same manner as the standardized Kirby-Bauer procedure used for less fastidious organisms, except that supplemented Mueller-Hinton agar plates were incubated in an increased-CO2 atmosphere. Using this method, ampicillin-susceptible strains of Haemophilus produced zone diameters of .gtoreq. 22 mm, while ampicillin-resistant strains produced zones of .ltoreq. 18 mm. All strains were chloramphenicol-susceptible and produced zone diameters of .gtoreq. 30 mm. This method would allow routine disk-diffusion testing of isolates of H. influenzae by hospital diagnostic laboratories, using a clear medium that closely resembles unsupplemented Mueller-Hinton agar.