The DUEL study: a multi-center in vitro evaluation of linezolid compared with other antibiotics in the Netherlands

Objective   To evaluate bacterial susceptibility to linezolid in the Netherlands in comparison with other antibiotics.
Methods   Bacterial strains were isolated between September 1999 and January 2000 from patients presumed to require antibiotic treatment. The in vitro activity of 1226 strains from 34 participating laboratories was tested against linezolid, vancomycin, teicoplanin, oxacillin, penicillin, erythromycin, ampicillin and other antibiotics against enterococci, coagulase-negative staphylococci, Staphylococcus aureus and Streptococcus pneumoniae . Minimal inhibitory concentrations (MIC) were obtained with the E test on Mueller-Hinton agar: every laboratory included control strains. For vancomycin and teicoplanin only, brain–heart infusion agar and an inoculum of 2.0 McFarland was used for Staphylococcus aureus , coagulase-negative staphylococci and enterococci to support a better growth and clear recognition of hetero-resistant colonies.
Results   The values of MIC₉₀ for linezolid were 1.5, 0.75, 0.75 and 1 mg/L for Staphylococcus aureus , coagulase-negative staphylococci, Streptococcus pneumoniae and enterococci, respectively. Six enterococcal strains with decreased susceptibility against vancomycin or teicoplanin were identified as Enterococcus faecium , E. gallinarum and E. casseliflavus (two strains each) and they were found to harbor vanA , vanC1 and vanC2/3 genes, respectively. Nine per cent of Streptococcus pneumoniae (an increase from 1% 4 years ago) showed decreased susceptibility to erythromycin, of both the ermB and mefE type; there was no cross-resistance with linezolid. Twelve coagulase-negative staphylococcal strains were resistant to teicoplanin.
Conclusion   Linezolid is a promising drug in the treatment of infections caused by Gram-positive cocci. Cross-resistance with other antibiotics tested was not found.