COMPARISON OF AGAR DILUTION, BROTH MICRODILUTION, DISK DIFFUSION, E-TEST, AND BACTEC RADIOMETRIC METHODS FOR ANTIMICROBIAL SUSCEPTIBILITY TESTING OF CLINICAL ISOLATES OF THE NOCARDIA-ASTEROIDES COMPLEX

An evaluation was undertaken to determine the optimal method for the i n vitro susceptibility testing of 26 Nocardia asteroides complex isola tes to the following antimicrobial agents: amikacin, ampicillin, amoxi cillin-clavulanate, ceftriaxone, ciprofloxacin, erythromycin, imipenem , minocycline, and trimethoprim-sulfamethoxazole, Five testing methods were studied including the agar dilution, broth microdilution, and di sk diffusion methods, the epsilometer test (E-test), and the BACTEC ra diometric method. Results for each antimicrobial agent and each testin g method were interpreted as indicating susceptibility, intermediate s usceptibility, or resistance according to current guidelines of the Na tional Committee for Clinical Laboratory Standards (NCCLS) for bacteri a that grow aerobically and were then compared to a ''gold standard'' susceptibility test result, The gold standard result for each Nocardia isolate was established by a consensus of the results of the majority of testing methods used in the study, When the results were combined for all antimicrobial agents tested against all Nocardia isolates by a ll methods, the BACTEC radiometric method produced the highest level o f agreement (97.9%) with the consensus results and had the fewest very major (n = 1), major (n = 2), and minor (n = 2) errors. In contrast, the results of the agar dilution method were in least agreement (93.2% ) with the consensus results, and this method also produced the most v ery major (n = 8), major (n = 4), and, along with the disk diffusion m ethod, minor (n = 6) errors, For all test methods, interpretive errors were most frequent when testing ampicillin or amoxicillin-clavulanate . Moreover, for all Nocardia nova isolates tested, ampicillin suscepti bility results by any of the testing methods were not in agreement wit h the results of testing for beta-lactamase by the nitrocefin (Cefinas e) disk method, We conclude that among the methods evaluated, the BACT EC radiometric method appeared to be the best for determining the in v itro susceptibilities of members of the N. asteroides complex to a pan el of nine antimicrobial agents, However, none of the test methods, in cluding the BACTEC method, accurately predicted the ampicillin resista nce of the N. nova isolates tested, all of which produced beta-lactama se. Presuming that this beta-lactamase hydrolyzes ampicillin, this dis parity may relate to the NCCLS breakpoints that were used, which may r equire modification for this antimicrobial agent when tested against N . nova isolates.