High proportion of pharyngeal carriers of commensal streptococci resistantto erythromycin in Spanish adults

The presence of erythromycin-resistant (ErR) commensal streptococci in the throat of 110 healthy subjects and 87 patients with pharyngitis was investi gated. The resistance determinants were studied by PCR using the primers fo r mef and erm genes, followed by hybridization and sequencing analysis. Ove rall, 94.4% of the subjects carried one or more ErR strains in their pharyn x. A total of 253 ErR strains was studied: 127 (50.2%) showed constitutive or inducible resistance to clindamycin (MLSB phenotype) and 126 (49.8%) wer e susceptible to clindamycin (M phenotype). In 50 subjects (25.4%) both phe notypes were detected. The ermB gene was predominant among the MLSB phenoty pe strains (97.6%). The mefA (mefA/mefE) gene was detected in 100% of the s trains with the M phenotype. One Streptococcus oralis strain bearing the ML SB phenotype carried both mefA and ermB genes. The mefA gene from clinical isolates of Streptococcus mitis and S. oralis was transferred by conjugatio n to an erythromycin-susceptible Streptococcus pneumoniae strain.