[Study on susceptibilities of freshly isolated strains to macrolide antibiotics].

Macrolide (ML) susceptibilities of clinically isolated bacteria obtained freshly during 1989 were determined and year to year differences in ML-resistances of the clinical isolates in our laboratory were examined with epidemiological consideration in mind. 1. The distribution of minimum inhibitory concentrations (MICs) of MLs against isolates of Staphylococcus aureus showed 2 peaks, one peak representing S. aureus isolates susceptible to MLs and the other peak representing those resistant to MLs. Proportions of resistant isolates to various MLs were 36% to erythromycin, 24% each to kitasamycin, josamycin, midecamycin and midecamycin acetate. MICs of rokitamycin (RKM), however, distributed themselves into 3 peaks of less than or equal to 0.39 micrograms/ml (76%), 1.56-3.13 micrograms/ml (12%) and greater than or equal to 100 micrograms/ml (12%). Staphylococcus spp. including Staphylococcus epidermidis showed different resistance characteristics to 14-membered ring MLs and to 16-membered ring MLs. Further, the proportion of resistant organisms to RKM was the smallest among proportions of resistant organisms to various 16-membered ring MLs tested. These differences in proportions of resistant organisms to different MLs may have resulted from differences in resistance induction activities of different MLs. The above described ratios of resistant organisms to MLs among S. aureus did not vary since 1975 among clinical isolates tested in our laboratory. 2. Four per cent (4%) of Streptococcus pyogenes isolates were MLs resistant, and 18% of Streptococcus pneumoniae isolates were resistant to MLs. Among the isolates tested in our laboratory since 1985, resistant isolates of S. pyogenes are clearly decreasing and those of S. pneumoniae seem to be increasing.(ABSTRACT TRUNCATED AT 250 WORDS)