Disk Diffusion Susceptibility Testing of Nocardia Species

The effectiveness of 13 antimicrobial agents against 51 clinical isolates of Nocardia was determined with use of agar dilutions and disk diffusion method. Amikacin inhibited less than 90% of isolates and, like the other aminoglycosides, showed good correlation between minimal inhibitory concentrations and sizes of zones of inhibition around the disks. Both sulfisoxazole and trimethoprim-sulfamethoxazole were very active, although they required a 2- to 3-log lower inoculum for demonstration of susceptibility. Results with the two sulfa disks were variable, but they did allow distinction between sensitive and intermediate strains. All of the isolates of Nocardia were inhibited by 6.3 microng of minocycline; however, the degree of susceptibility could not be determined by zone diameters. Only two-thirds of these clinical isolates of Nocardia grew rapidly enough to be assayed by either susceptibility method.