[Different phenotypes of erythromycin-resistant blood isolates in viridans group streptococci]

Macrolide resistance has been widely studied in Streptococcus pyogenes and Streptococcus pneumoniae although not in viridans group streptococci (VGS). We studied 30 blood culture isolates of viridans group streptococci (25 resistant to erythromycin: 10 S. mitis, 8 S. milleri, 6 S. sanguis and 1 S. salivarius; and 5 susceptible: 2 S. mitis, 2 S. milleri and 1 S. sanguis). We carried out a double-disk test and determined MICs. The susceptibility testing was carried out by agar dilution for 14-, 15- and 16-member lactone ring macrolides, as well as for clindamycin and quinupristin-dalfopristin. Fifty-six percent of the erithromycin-resistant strains (6 S. mitis, 6 S. milleri and 2 S. sanguis) showed an MLS(B) phenotype, with a high level of intrinsic resistance to all the macrolides studied and clindomycin; 28% were of the M phenotype (4 S. sanguis, 2 S. mitis and 1 S. salivarius). We found a third resistance phenotype, which was present in 4 strains with MICs of 2-8 µg/ml, with resistance to macrolides and different degrees of resistance to clindamycin. All isolates were fully susceptible to quinupristin-dalfopristin. The MLS(B) and M phenotypes initially described in S. pyogenes and S. pneumoniae are also observed in VGS.