Abstract
In the one-year period between June 2003 and May 2004, 1,052 infants and children under 15 years of age with acute otitis media or acute rhinosinusitis visited our hospital. From these patients, 1,322 strains of Streptococcus pneumoniae were isolated, and tested for antimicrobial susceptibility by the broth microdilution method, as well as for mutations in the penicillin-binding protein (pbp) gene and the macrolide-resistance genes by PCR. According to the CLSI guidelines, 145 (11.0%), 544 (41.1%), and 633 (47.9%) strains were classified as PSSP, PISP, and PRSP, respectively, and 59 (4.5%), 391 (29.6%), and 872 (65.9%) strains were classified as gPSSP, gPISP, and gPRSP, respectively. We identified the macrolide-resistance genes mefA and ermB in 447 (33.8%) and 593 (44.9%) strains, respectively, both genes in 97 strains (7.3%), and neither of them in 185 strains (14.0%). The number of pbp gene mutations increased with the percentage of strains with mutations in macrolide-resistance genes. In terms of MIC90, MERP and VCM (0.5 μg/mL) were most active, followed by CTX, CDTR, and LVFX (1.0 μg/mL), then by PCG, CFPM, CZOP, and CVA/AMPC (2.0 μg/mL). The MIC90s of other antimicrobials were all 4.0 μg/mL, indicating the emergence of significant resistance mutations. The relationships between pbp gene mutations and drug susceptibility in terms of MIC90s were as follows. The MIC90 of PCG for gPSSP was 0.12 μg/mL, that for gPISP ranged from 0.12 μg/mL to 2.0 μg/mL according to the number of pbp gene mutations, and that for gPRSP was 4.0 μg/mL, indicating that a greater number of pbp gene mutations are associated with a higher value of MIC90. The MIC values of other β–lactam antibiotics showed similar tendencies. The MIC90 of CAM for strains with no macrolide−resistance gene was 0.12 μg/mL, and those for strains with the mefA or ermB gene or both were more than 2 μg/mL. The MIC90s of CLDM for strains with the mefA gene or no resistance gene were 0.12 μg/mL, and those for strains with the ermB gene or both the mefA and ermB genes were 1 μg/mL. Thus, there was little difference in the frequency of resistance of S. pneumoniae between the classification under the CLSI guidelines and that based on pbp gene mutations identified by PCR. Susceptibility to β–lactam antibiotics was lower overall than that reported previously. With the increase in pbp gene mutations, resistance to β–lactam antibiotics has increased, resulting in decreased susceptibility to CAM and CLDM. Therefore, further surveillance is needed of antimicrobial resistance by drug sensitivity testing and genetic analysis using PCR.
