EVALUATION OF A DISK DIFFUSION METHOD FOR DETERMINING SUSCEPTIBILITY OF MYCOBACTERIUM-AVIUM COMPLEX TO CLARITHROMYCIN

We evaluated an agar disk diffusion method for determining the suscept ibility of Mycobacterium avium complex to clarithromycin. Isolates wer e inoculated onto the surface of a Middlebrook 7H11 plate, followed by the application of a 15-mu g clarithromycin disk. Zone sizes were rea d after 5-7 days of incubation. Zone sizes had a bimodal distribution; 40 isolates (10%) had no zone of inhibition, whereas the zone sizes f or the remaining isolates ranged from 11 to 60 mm. Most isolates (37/4 0) having no zone of inhibition came from patients who had been treate d previously with clarithromycin. Fifty-one isolates were also tested for clarithromycin susceptibility using a microdilution broth method. Defining susceptibility as a zone size of >10 mm, disk diffusion test results agreed with the results by the microdilution broth method for 50 of 51 (98%) isolates tested by both methods. Agar disk diffusion is a promising method for the determination of clarithromycin susceptibi lity testing for M. avium complex. (C) 1998 Elsevier Science Inc.