We examined antibacterial activities of 4 kinds of macrolides, erythromycin (EM), clarithromycin (CAM), azithromycin (AZM) and rokitamycin (RKM), against 6 bacterial species of clinical strains isoleted in 2002. Bacterial isolates used were each 50 strains of methicillin-susceptibleStaphylococcus aureus (MSSA), Streptococcus pyogenes, Streptococcus agalactiae, Moraxella (Branhamella) catarrhalis, Haemophilus influenzae and 43 strains of Streptococcus pneumoniae. S. agalactiae were derived from gynecological samples, and other species were isolated from respiratory specimens.
Antimicrobial activities againstS. aureus, S. pyogenes, S. agalactiae, M. catarrhalis and H. influenzae of 14-membered macrolides, such as EM and CAM, were higher than those of 16-membered macrolide, RKM. By contrast, againstS. pneumoniae, RKM was more effective than 14-membered macrolides.
Six, three and four strains ofS. aureus, S. pyogenes and S. agalactiae, respectively, were resistant to macrolides. Thirty-five among 43 pneumococcal isolates were resistant, and 15 of the 35 were highly-resistant, MIC of>128μg/ml, to any one of EM, CAM or AZM. Isolation frequency of resistant strains to RKM was lower than those to 14- and 15-membered macrolides: only one strain was highly-resistant and 12 were intermediately-resistant. No resistant strain was recognized inM. catarrhalis and H. influenzae.
Further, we analyzed the resistant mechanisms, methylation or efflux, of macrolide resistant strains by the double-disk method. Methylation was major mechanism inS. aureus, and inS. pyogenes, all of the resistance was caused by methylation. In S. agalactiae andS. pneumoniae, methylation and efflux shared about half and half.

View full abstract

A total of 2865 strains of the causative organisms isolated from the patients with acute pharyngitis and tonsillitis at the primary medical institutions were used in this study. The MICs of levofloxacin (LVFX) and other oral antimicrobial drugs were determined and evaluated by the NCCLS guideline.
LVFX, cefditoren (CDTR) and cefcapene (CFPN) were potently active against 773 isolates ofHemophilus influenzae, the MIC₅₀s of LVFX being≤0.06μg/mL and also the same as the MIC₉₀s of LVFX. LVFX was the most active against 496 isolates of Enterobacteriaceae. The MIC₅₀s of LVFX were≤0.06μg/mL and were lower than those of CDTR, cefdinir (CFDN) and cefpodoxime (CPDX)(MIC₅₀s: 0.5, μg/mL).
The MIC₉₀s of these cephems were markedly higher than the respective MIC₅₀s, whereas MIC₅₀ of LVFX was 0.12μg/mL, only twice the MIC₅₀. Against the majority of Streptococcus pyogenes (555 isolates) and Strep-tococcus spp.(495 isolates), CDTR, CFDN, CPDX and CFPN were highly active (MICs: ≤0.06μg/mL), and clarithromycin (CAM) and azithromycin (AZM) were also active against these organisms (MICs: 0.12 to 0.25μg/mL). Against S. pneumoniae (92 isolates), CDTR and CFDN were active (MIC₅₀s: 0.12 and 0.25μg/mL, respec-tively). However, the MIC₉₀s of these drugs were 4-8 times the MIC₅₀s. Against Moraxella (Branhamella) catarrhalis (454 isolates), LVFX was potently active, the MIC₉₀ of LVFX being≤0.06μg/mL and MIC₉₀s of the other cephems being 0.5μg/mL or more.
When the susceptibility of these strains to LVFX was evaluated by the NCCLS guideline, about 3% of other Streptococcus spp. were resistant to the drug but no test strains resistant to LVFX were detected in H.influenzae, S.pyogenes or Enterobacteriaceae. On the other hand, the percentages of strains susceptible to the cephems tested were 60-90%, which were quite different according to kinds of drugs and species used. Furthermore, the strains of S. pneumoniae resistant to CFDN and CPDX, and those to CAM and AZM were 21-25% and 50% or more, respectively, whereas no LUFX-resistant strains were detected.
The major pathogens isolated from patients with pharyngitis and tonsillitis in the primary institutions were highly susceptible to LVFX. These results suggest that LVFX is a useful drug which is potently active against the strains resistant to oral cephem and macrolide antibiotics.

View full abstract