High prevalence of erythromycin-resistant and clindamycin-susceptible (M phenotype) viridans group streptococci from pharyngeal samples: a reservoir of mef genes in commensal bacteria

Sir, An increase in the isolation of Streptococcus pyogenes resistant to macrolides, often mediated by the recently described M phenotype, has been reported in several countries. Strains of the M phenotype carry the mef(A) gene, which confers resistance to 14and 15-, but not 16-membered macrolides or clindamycin. Pneumococcal resistance to macrolides mediated by the mef(A) gene has also been described. Some strains of viridans group streptococci (VGS) carry the mef(A) gene. This ubiquitous group of bacteria frequently colonize the oropharynx in healthy individuals, and may thus be a reservoir for the dissemination of the mef(A) gene to S. pyogenes and Streptococcus pneumoniae. To ascertain the prevalence of VGS with the M phenotype, we studied strains of VGS isolated from pharyngeal exudates from patients with symptoms of pharyngitis and from healthy people. The prevalence of macrolide resistance and the different phenotypes were determined and, in some strains, the genes encoding resistance were characterized. During 1999, 198 pharyngeal exudates obtained from outpatients in our Health Authority Area of Madrid, Spain, were studied. Most samples were from patients with symptoms of pharyngitis while 17 were from post-treatment controls of group A -haemolytic streptococcal infection. All the samples were from children aged 14 years. In the same period, pharyngeal exudates from 50 healthy people in the same Health Authority Area (27 children and 23 adults), who had not received antibiotic treatment in the previous 3 months, were also collected. Samples were plated on Columbia sheep blood agar and -haemolytic Gram-positive cocci that gave a negative catalase test were studied further. All colonies of each sample with different morphotypes (determined by lens examination) were studied. The optochin test was performed to differentiate VGS from S. pneumoniae. Antimicrobial susceptibility testing was performed by the agar dilution method according to the guidelines of the NCCLS. Antibiotics were obtained as powders of known potency from Sigma Chemical Co., St Louis, MO, USA (penicillin G, erythromycin and clindamycin), Pfizer Inc., New York, NY, USA (azithromycin) and Menarini, Barcelona, Spain (diacetil–midekamycin miocamycin). Plates were incubated overnight at 35 C in an atmosphere containing 5% CO2. The range of interpretative categories for each antibiotic were those recommended by the NCCLS in the 2000 supplement. The MIC breakpoint for miocamycin resistance was 4 mg/L, as defined by the Comité de l’Antibiogramme de la Société Française de Microbiologie. Different macrolide resistance phenotypes were identified using discs containing erythromycin (15 g) or clindamycin (2 g) as described by Seppälä et al. The resistance genes of 50 erythromycin-resistant strains (40 with the M phenotype and 10 with the MLSB phenotype) from both healthy and symptomatic people were characterized by PCR amplification of erm genes, using degenerate erm primers and by PCR amplification of mef(A)/(E) genes using specific primers. VGS strains with a positive PCR with mef(A)/(E) primers were considered to have the mef(A) gene, following recent recommendations. The 2 test was used for statistical analysis. The 198 pharyngeal exudates from patients with pharyngitis yielded 296 VGS strains (1.49 strains per exudate) while the pharyngeal exudates from healthy people yielded 71 VGS strains (1.42 strains per exudate). The Table shows the susceptibility of VGS obtained in the three groups studied (healthy children, healthy adults, and patients with symptoms of pharyngoamigdalitis or posttreatment or post-infection controls). Among the isolates from symptomatic patients, 23.6% (70 isolates) were susceptible to erythromycin, 60.8% (180 isolates from 115 patients) had the M phenotype and 15.6% (44 isolates, constitutive; two isolates, inducible) showed the MLSB phenotype. Among the isolates from healthy children and adults, 36.6% (26 isolates from 17 patients) were susceptible to erythromycin, 50.7% (39 isolates from 30 patients) had the M phenotype and 12.7% (six isolates from five patients) had the MLSB constitutive phenotype. When we compared the strains resistant and intermediate Correspondence Journal of Antimicrobial Chemotherapy (2001) 48, 587–595