Studies were made on 66 strains of Streptococcus pneumoniae which were obtained from clinical specimens in 1991 through 1993 and showed 19 mm or less of disk inhibition zone diameter (DIZD) against 1 microgram oxacillin (MPIPC) disk. The studies included the determination of their serotypes, antimicrobial susceptibility, and comparison between microbroth dilution (MD) method and Kirby-Bauer (K-B) method. In the study of distribution of serotypes, additional 32 strains which showed 20 mm or more in DIZD were included for study. The results were as follows. 1) About 70% of 98 strains of S. pneumoniae were serotyped by 6 kinds of antisera. Among penicillin-susceptible S. pneumoniae (PSSP), type 3 were 20.6%, type 19, 15.9%, type 6, 14.3%, type 18, 9.5%, type 14, 7.9%, and type 4, 1.6%. Among penicillin-intermediate S. pneumoniae (PISP), and penicillin-resistant S. pneumoniae (PRSP), type 19 were 60%, and type 18, 8.6%. In PISP and PRSP, more than half were type 19, which indicates they are distinctly different from PSSP in serotypical distribution. 2) As to the difference between screening by MPIPC disk and minimum inhibitory concentration (MIC) by benzylpenicillin (PCG), among 66 MPIPC resistant strains, PSSP strains were 31 in number (47%). 3) MIC showed that PISP and PRSP strains were more resistant than PSSP against cefaclor (CCL), cefazolin (CEZ), cefotiam (CTM), cefotaxime (CTX), imipenem (IPM), minocycline (MINO), and erythromycin (EM), but no difference was found in the 2 groups of strains in MIC with clindamycin (CLDM) and ofloxacin (OFLX). 4) All type 3 strains formed mucoid colonies and were resistant to MINO and highly resistant to EM and CLDM. 5) By NCCLS, category of antimicrobial susceptibility is determined against CCL, EM, OFLX, in MD method and K-B method. Against these antibiotics, the complete agreement rates were 75.8%, 92.4% and 86.4% respectively.
