Detection of macrolide resistance in Streptococcus pneumoniae

We determined the susceptibilities of recent clinical isolates of Streptococcus pneumoniae to 19 antibiotics. The frequency of erythromycin nonsusceptibility was high, i.e. 8/13 (61.5%), 10/14 (71.4%) and 11/11 isolates (100%) from 13 penicillin-susceptible, 14 penicillin-intermediate and 11 penicillin-resistant S. pneumoniae, respectively. Macrolide resistance was detected by polymerase chain reaction (PCR) and disk diffusion methods. Of these erythromycin-nonsusceptible pneumococcal isolates, 13/29 (44.8%) isolates contained genomic copies of MEFA and showed non-'D'-shaped zones of inhibition observed around rokitamycin and/or clindamycin disks. Sixteen out of 29 isolates (55.2%) contained copies of ERMB and showed typical 'D'-shaped zones of inhibition, except one isolate. Although the macrolide resistance determinants, MEFA and ERMB, could be identified by PCR and disk diffusion methods, PCR methods were more reliable in elucidating these determinants. The susceptibility pattern to 14-, 15- and 16-membered macrolides and clindamycin differed between the MEFA+ and ERMB+ isolates. All isolates were susceptible to levofloxacin, sparfloxacin and vancomycin. The MICs of sitafloxacin were lowest among the fluoroquinolones examined for 38 isolates.