In vitro activity of linezolid (U-100766) against Haemophilus influenzae measured by three different susceptibility testing methods

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Linezolid has excellent antibacterial activity against a wide range of Gram-positive organisms. Early in vitro investigations suggested that the compound also had activity against some Gram-negative species, including those commonly associated with community-acquired respiratory tract infections ( Haemophilus influenzae and Moraxella catarrhalis). Against 603 recent clinical isolates of H. influenzae from geographically diverse regions of the world, tested by the reference broth microdilution method (HTM), linezolid MIC values ranged from 2–64 μg/ml (MIC 50 and MIC 90 at 16 μg/ml and 32 μg/ml, respectively). A subset of 328 strains was also tested to compare broth microdilution and Etest (AB BIODISK, Solna, Sweden) methodologies. The Etest method produced slightly higher MIC results attributable to a growth-enhancing effect of the 5% CO 2 incubation used in the test procedure and pH changes. Linezolid activity versus H. influenzae was limited as evidenced by reference test results (susceptible breakpoints at ≤2 or ≤4 μg/ml) and variable endpoints were obtained when alternative methods were used such as the Etest or standardized disk diffusion procedures. Clinical laboratories should limit the testing of linezolid against Gram-negative species ( H. influenzae).