Susceptibility testing of Nocardia species for the clinical laboratory

Not all patients are able to tolerate or show a favorable response to the treatment of choice for Nocardia, the sulfonamides. Many new drugs are available with good activity against N. asteroides, the most common pathogenic species, although susceptibility to these agents, including amikacin, amoxicillin/clavulanic acid, and the third generation cephalosporins, is variable. For these and other reasons, we recommend routine susceptibility testing of Nocardia. Disk diffusion testing on Mueller Hinton agar is the best currently available clinical method. A suggested control strain and tentative susceptible and resistant breakpoints for 12 antimicrobial agents are provided. This includes agents not previously evaluated, including cefotaxime, amikacin, ciprofloxacin, and doxycycline. The zones of inhibition were all larger than those currently used by the NCCLS for rapidly growing bacteria, and the disk breakpoints generally fit best with the MIC breakpoints used with the dilution susceptibility method (M7-T). A broth microdilution MIC method is described that showed good correlation with disk diffusion results, but need additional study. Because of limited experience in most laboratories with this species, reliance on a good reference laboratory for confirmatory susceptibility testing is recommended. Beta-lactamase testing is not helpful, as almost all Nocardia produce beta-lactamase, although many isolates retain susceptibility to selected beta-lactams.