Susceptibility to quinupristin-dalfopristin and linezolid in 839 clinical isolates of gram-positive cocci from Norway

A total of 839 clinical isolates of Gram-positive cocci from Norway including Staphylococcus aureus (n=214), coagulase negative Staphylococcus spp. (n=100), Streptococcus pyogenes (n=99), Streptococcus agalactiae (n=80), Streptococcus pneumoniae (n=127), Streptococcus spp. viridans group (n=70), Enterococcus faecalis (n=75), and Enterococcus faecium (n=74), were tested by E-test for susceptibility to a range of antimicrobials including the novel antibiotics quinupristin-dalfopristin and linezolid. Subgroups of oxacillin resistant S. aureus and coagulase negative Staphylococcus spp., penicillin non-susceptible S. pneumoniae and vancomycin resistant Enterococcus spp. were specifically included as they are the intended targets for these new drugs. All isolates were susceptible to linezolid (MIC50 and MIC90 0.25–2.0 mg/l, MIC range 0.12–2 mg/l). Staphylococcal and streptococcal isolates were also susceptible to quinupristin-dalfopristin except for some intermediately susceptible viridans group isolates (MIC50 and MIC90 0.25–2 mg/l, MIC range 0.125–2 mg/l). Enterococcus faecium (MIC90=4.0 mg/l) and Enterococcus faecalis (MIC50=8.0 mg/l, MIC90≥32 mg/l) were less susceptible to this substance. There was no linkage between reduced susceptibility to linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. The study demonstrated a high prevalence of in vitro susceptibility to linezolid and quinupristin-dalfopristin, which is necessary for their use in the treatment of infections with resistant Gram-positive pathogens. The results were used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to quinupristin-dalfopristin and linezolid in Norway.