Use of clindamycin disks to detect macrolide resistance mediated by ermB and mefE in Streptococcus pneumoniae isolates from adults and children

We studied 198 macrolide-resistant S. pneumoniae isolates obtained from adu lts and children to evaluate whether 2-mu g clindamycin disks can distingui sh between isolates manifesting ermB- versus mefE-mediated resistance to cl arithromycin and to determine the relative frequency with which each resist ance mechanism occurred in these populations. The mefE gene was predominant among 109 isolates from children, occurring in 73.4% versus 50.6% of 89 is olates from adults. Three isolates (1.5%) did not amplify either gene. Amon g 125 mefE(+) isolates, the MIC of clarithromycin at which 90% of the isola tes tested were inhibited, determined by Etest, was 32 mu g/ml versus >256 mu g/ml in 70 ermB(+) isolates, All ermB(+) isolates were highly resistant to clindamycin (MICs >256 mu g/ml), whereas all mefE(+) isolates were susce ptible to clindamycin using the 2-mu g disk. Testing S. pneumoniae from the respiratory tract for susceptibility to clindamycin by agar disk diffusion is an easy and inexpensive method to estimate the frequency of resistance mediated by ermB in specific patient populations, Macrolide resistance medi ated by ermB is usually of greater magnitude than that due to mefE. Clinica l studies are needed to determine the significance of high- versus low-leve l macrolide resistance in S. pneumoniae.