Detection of penicillin and extended-spectrum cephalosporin resistance among Streptococcus pneumoniae clinical isolates by use of the E test

Increasing penicillin resistance and the initial recognition of resistance to extended-spectrum cephalosporins among Streptococcus pneumoniae isolates have placed greater emphasis on accurate methods for susceptibility testing of clinical isolates. This study has evaluated the use of the E test (AB Biodisk NA, Piscataway, N.J.) for the detection of penicillin and cefotaxime resistance among 147 pneumococcal clinical isolates in three geographically separate laboratories. These included 42 penicillin-resistant (MIC, > or = 2 micrograms/ml) and 14 cefotaxime-resistant (defined here as an MIC of > or = 2 micrograms/ml) isolates. E test strips were applied to the surface of Mueller-Hinton sheep blood agar plates and incubated at 35 degrees C in 5% CO2 for 20 to 24 h. E test MICs were compared with MICs determined with lysed horse blood-supplemented Mueller-Hinton broth in a microdilution format as recommended by the National Committee for Clinical Laboratory Standards. Penicillin MICs agreed within one log2 dilution for 136 of 147 (92.5%) isolates, and cefotaxime MICs agreed within one log2 dilution for 142 of 147 (96.6%) isolates. No very major or major interpretive errors occurred with either penicillin or cefotaxime E test MIC results. There were 9.5 and 5.4% minor interpretive category errors with penicillin and cefotaxime E test MICs, respectively. These data indicate that the E test represents a convenient and reliable method for the detection of penicillin or cephalosporin resistance in pneumococci.