Phenotypic differentiation of macrolide resistance among streptococcus pneumoniae carrying mefA and/or ermB genes

We examined whether 55 isolates of erythromycin-resistant Streptococcus pneumoniae (MIC ≧1 μg/mL) carrying mefA and/or ermB genes would develop resistance to telithromycin. Fifteen isolates, carrying only the mefA gene, had the M resistance phenotype by their susceptibility pattern and their susceptibility to rokitamycin did not change after exposure to 0.1 μg/mL of erythromycin. Of the other 40 isolates, 25 carried the ermB gene and 15 carried the both mefA and ermB genes, all of them showed high resistance to clindamycin (MIC ≧ 128 μg/mL) and were resistant to macrolides, lincosamides and streptogramin B (MLS_B resistance phenotype). Twenty six isolates with the MLS_B resistance phenotype showed decreased sensitivity to telithromycin after exposure to erythromycin, although the elevated MICs of telithromycin remained below 1 μg/mL, the remaining 14 isolates did not showed an obvious decrease of their sensitivity to telithromycin (only two-fold dilution). However, by adding the telithromycin disk to the erythromycin- rokitamycin double disk diffusion test, the zone of inhibition around the telithromycin disk was blunted proximal to the erythromycin disk in all isolates with the MLS_B resistance phenotype including the 25 isolates which were considered to have a true cMLS_B phenotype because of their constitutive resistance to rokitamycin (MIC ≧ 4 μg/mL). Furthermore, in 16 isolates, highly resistance to rokitamycin (MIC≧64 μg/mL), the zone of inhibition around the telithromycin disk was blunted to both erythromycin and rokitamycin disks. These results indicate that the expression of telithromycin resistance was induced even in S. pneumoniae isolates with a true cMLS _B resistance phenotype.