In vitro activity of linezolid, clarithromycin and moxifloxacin against clinical isolates of Mycobacterium kansasii

Objectives: To compare the activity of linezolid with a range of drugs used in the treatment of Mycobacterium kansasii infections. Methods: The in vitro activity of clarithromycin, ethambutol, isoniazid, linezolid, moxifloxacin and rifampicin against 104 M. kansasii clinical isolates was studied by a broth-microdilution method. Results: The percentages of resistant isolates against isoniazid, rifampicin and ethambutol were 2.9%, 1.9% and 2.9%, respectively. All isolates were susceptible to clarithromycin and moxifloxacin both with MIC90 values of 0.125 mg/L. Linezolid was active against all isolates with MIC50 and MIC90 values of 0.5 and 1 mg/L, respectively, both below the susceptibility breakpoint established for mycobacteria. Conclusion: Linezolid, clarithromycin or moxifloxacin, could be used as alternative drugs for treatment of infections due to rifampicin-resistant isolates as well as short-course or intermittent therapy of M. kansasii lung disease.