Activities of Linezolid against Rapidly Growing Mycobacteria

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Abstract
Linezolid is an oxazolidinone available as an oral drug which has activity against most gram-positive bacteria. However, few species of the genusMycobacteriumhave been studied. We tested 249 clinical isolates and 10 reference strains of rapidly growing mycobacteria for susceptibility to linezolid by broth microdilution. Clinical species included theMycobacterium fortuitumgroup (n= 74),M. abscessus(n= 98),M. chelonae(n= 50),M. mucogenicum(n= 10), andM. fortuitumthird biovariant complex (10). The modal MIC forM. mucogenicumwas 1.0 μg/ml, and the MIC at which 90% of the isolates tested are inhibited (MIC90) was 4 μg/ml; the modal MIC for theM. fortuitumgroup was 4 μg/ml, and the MIC90was 16 μg/ml; the modal MIC for theM. fortuitumthird biovariant complex was 4 μg/ml, and the MIC90was 8 μg/ml; the modal MIC forM. chelonaewas 8 μg/ml, and the MIC90was 16 μg/ml; and the modal MIC forM. abscessuswas 32 μg/ml, and the MIC90was 64 μg/ml. Based on peak levels of linezolid in serum of 15 to 20 μg/ml, we propose the following broth MIC breakpoints for these species: susceptible, ≤ 8 μg/ml; moderately susceptible, 16 μg/ml; and resistant, ≥32 μg/ml). These studies demonstrate the excellent potential of linezolid for therapy of rapidly growing mycobacteria.

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