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

Abstract

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.