Activity of linezolid against Gram-positive cocci isolated in French hospitals as determined by three in-vitro susceptibility testing methods.
In total, 844 strains of Gram-positive cocci were collected from six university hospitals in France between September 1999 and January 2000. MICs of linezolid were determined: (i) for all strains by agar dilution (method A); (ii) by broth microdilution (method B) for staphylococci and enterococci; (iii) by Etest (method E) for beta-haemolytic streptococci and Streptococcus pneumoniae. Susceptibility to other antibiotics was determined by the disk diffusion method. MIC50 and MIC90 values were identical (2 mg/L) for methicillin-susceptible Staphylococcus aureus (n = 179) by methods A and B. Linezolid was active against methicillin-resistant S. aureus (n = 117), with an MIC90 of 2 mg/L (methods A and B), but with a lower MIC50 of 1 mg/L by method A. Of the 200 coagulase-negative staphylococci, 56.5% were methicillin-resistant and 43.5% were methicillin-susceptible. Linezolid had similar in-vitro activity by methods A and B (MIC50 and MIC90 values of 1-2 mg/L), irrespective of methicillin susceptibility. The MIC90 of linezolid for all enterococci (150 Enterococcus faecalis and 50 Enterococcus faecium) was 2 mg/L by both methods. MICs of linezolid for beta-haemolytic streptococci had a narrow range of 0.5-2 mg/L (method A) and 0.125-2 mg/L (method E). Pneumococci (n = 118), including 67 penicillin G-intermediate and -resistant strains, were all inhibited by linezolid 2 mg/L (MIC90 of 2 mg/L by methods A and E). No strain had an MIC of > 2 mg/L by agar dilution or Etest, or of > 4 mg/L by broth microdilution. Overall, the study confirmed the good in-vitro activity of linezolid and the very narrow range of MICs for Gram-positive cocci susceptible or resistant to other antibiotics, irrespective of the method used.
Laboratoire de Bactériologie, Groupe Hospitalier Bichat-Claide Bernard, Paris, France. claudette.muller@bch.ap-hop-paris.fr
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Muller-Serieys
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Drugeon
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Etienne
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Lascols
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Leclercq
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Nguyen
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CJ
Soussy
D000081:Acetamides / Q000494:pharmacology*; D000890:Anti-Infective Agents / Q000494:pharmacology*; D016983:Enterococcus / Q000187:drug effects; D006095:Gram-Positive Cocci / Q000187:drug effects*; D006801:Humans; D000069349:Linezolid; D008826:Microbial Sensitivity Tests / Q000379:methods*; D023303:Oxazolidinones / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects; D013291:Streptococcus / Q000187:drug effects
D000081:Acetamides; D000890:Anti-Infective Agents; D023303:Oxazolidinones; D000069349:Linezolid