EVALUATION OF COMMERCIAL METHODS FOR DETERMINING ANTIMICROBIAL SUSCEPTIBILITY OF STREPTOCOCCUS-PNEUMONIAE

Seven commercial systems for antimicrobial susceptibility testing of S treptococcus pneumoniae were evaluated by using a challenge set of 55 pneumococcal isolates with a variety of resistance phenotypes and geno types. Overall, the results produced by the Pasco and Etest methods we re found to be acceptable for all drugs tested except for trimethoprim -sulfamethoxazole testing by the Etest. The Just One system for penici llin MIC testing was also judged to be acceptable (minor error rate, 5 .5%), Although the Sensititre and MicroTech methods both produced 12.7 % minor errors with penicillin, the Sensititre method classified penic illin-intermediate strains as resistant or vice versa, while four of M icroTech's errors were among intermediate strains that were classified as susceptible. The MicroMedia (minor error rate, 16.4%) and MicroSca n Rapid (minor error rate, 63.6%) methods produced unacceptably high l evels of errors when testing penicillin, Minor error rates for cefotax ime and ceftriaxone ranged from a low of 12.7% (Etest and Sensititre) to a high of 28% (MicroMedia). Error rates were low for erythromycin, tetracycline, and chloramphenicol by most methods with the exception o f the MicroScan method, which had a high very major error rate for ery thromycin (34.6%), For testing of beta-lactam drugs, the Pasco, Etest, and Just One tests for penicillin are the most accurate methods; the Sensititre method also provided acceptable results.