[Macrolide resistance phenotypes and genotypes in group G streptococci]

Group G streptococci (GGS) are among the bacterial causes of pharyngitis. In this study, a total of 57 GGS strains isolated from throat swabs were initially screened by NCCLS disk diffusion test for penicillin and erythromycin susceptibilities. Phenotypes of the erythromycin resistance were determined by a triple-disk test and the responsible genes were sought by polymerase chain reaction (PCR) using mefA, ermA, ermA-subclass ermTR and ermB primer sets. Erythromycin and clindamycin minimal inhibitory concentrations (MIC) of the resistant isolates were measured by NCCLS agar dilution method, while susceptibility rates to some other antibiotics were determined by the disk diffusion test. All isolates were found susceptible to penicillin, and 8 (14%) were resistant to erythromycin. Of the resistant isolates, 3 (37.5%) expressed M phenotype and 5 (62.5%) had inducible macrolide-lincosamide-streptogramine B resistance (iMLS). All M isolates had mefA determinant. Of non-M isolates, 3 (37.5%) had subclass ermTR. In 2 (25%) isolates none of the four gene determinants was detected. MIC ranges of erythromycin and clindamycin for M and iMLS phenotypes were 4, 4-128 micrograms/ml and < or = 0.06-0.125, 0.25 - > or = 128 micrograms/ml, respectively. All the erythromycin-resistant isolates were also resistant to tetracycline. The high macrolide-resistance rate of GGS in our hospital deserves attention, however this finding should be confirmed by multi-centre studies including more representative isolates.