Antimicrobial susceptibility patterns and macrolide resistance genes of viridans group streptococci from normal flora

Objectives: Our aim was to study the antimicrobial susceptibilities and macrolide resistance mechanisms of viridans group streptococci isolated from the normal flora. Methods: In vitro susceptibilities of 16 antimicrobials were studied for 161 viridans streptococci (on average 5.8 isolates per person) from the normal flora of 28 elderly persons. Resistance mechanisms of erythromycin-resistant isolates were studied by the double disc test and PCR. Results: In all, 16.8% of the isolates were non-susceptible (MIC greater than or equal to 0.25 mg/L) to penicillin, but none showed high-level resistance (MIC greater than or equal to 4 mg/L). Resistance to erythromycin, tetracycline, quinupristin/dalfopristin, levofloxacin and moxifloxacin was found in 22.4, 27.3, 13.0, 1.9 and 1.9% of the isolates, respectively. Combined resistance to erythromycin and tetracycline was found in 13.0% of the isolates. Erythromycin-resistant isolates were isolated from 57% of the study persons. Of the erythromycin-resistant isolates 80.6% were of the M phenotype and 19.4% were of the macrolide-lincosamide-streptogramin B (MLSB) phenotype (one isolate with constitutive and six with inducible expression). Isolates with the M phenotype were the least susceptible to telithromycin, a new ketolide. The mef(A) gene was found in the isolates with the M phenotype and the erm(B) gene in the isolates with the MLSB phenotype. Conclusions: The distribution of phenotypes among the viridans streptococci resembles that found in Streptococcus pyogenes, with predominance of the M phenotype. However, the coding gene for the MLSB phenotype, erm(B), is the same in viridans streptococci as in Streptococcus pneumoniae. Viridans group streptococci carrying different resistance traits provide a pool of resistant bacteria that may transfer resistance determinants to more pathogenic organisms.