Oropharyngeal carriage of macrolide-resistant viridans group streptococci: a prevalence study among healthy adults in Belgium.

Viridans group streptococci (VGS) are gaining significance as reservoirs of resistance determinants for respiratory tract pathogens. Our aim was to investigate healthy adults for oropharyngeal carriage of VGS that are resistant to macrolides, as well as to other common antibiotics. Macrolide-resistant VGS were isolated from throat samples of 154 healthy Belgian adults, and phenotyped and genotyped for erm(A), erm(B) and mef(A). In vitro susceptibilities to 10 antimicrobials and the presence of tetracycline resistance genes were also determined. Carriage was detected in 71% of the population screened, from whom 157 unique, macrolide-resistant VGS were isolated. A constitutive (cMLS) phenotype was present in 105 isolates, of which 102 isolates carried either erm(B) or erm(B) + mef(A). The remaining three isolates did not present with any of the genes studied. All 45 isolates showing the M phenotype carried mef(A). The least abundant inducible (iMLS) isolates (n = 7) carried either erm(B) or erm(B) + mef(A). The most abundant macrolide-resistant VGS species was Streptococcus mitis (51%). Co-resistance to tetracycline was identified in 114 isolates, of which tet(M) was present in 105, tet(O) in two and both tet(M) and tet(O) in one, while the remaining six isolates did not present with either gene. tet(M) was also present in four tetracycline susceptible and two intermediately resistant isolates. Fluoroquinolone resistance (ciprofloxacin MIC > or = 4 mg/L) was detected in 16 isolates. Resistance to telithromycin, penicillin and chloramphenicol was appreciably low. This study highlights a high oropharyngeal carriage of macrolide-resistant VGS and its co-resistance to tetracycline and fluoroquinolones among healthy Belgian adults.