In vitro susceptibility of Mycobacterium kansasii to clarithromycin.

The MICs of the macrolide clarithromycin for 31 clinical isolates of Mycobacterium kansasii were determined by three different methods. The methods employed were the proportion resistance method on 7H10 agar, the radiometric (BACTEC) method, and the T100 method of datum analysis. All methods gave similar results. The MICs were in a narrow range from 0.16 to 0.50 microgram/ml, with the MICs for 90% of isolates tested of 0.50 microgram/ml for the agar dilution and radiometric methods and 0.37 microgram/ml for the T100 method. The MBCs were determined for nine representative isolates by the radiometric broth method. The MBCs were equal to the MICs for four isolates, and the MBCs were twofold higher than the MICs for five isolates. Killing of 99.9% of the bacterial population was achieved at a clarithromycin concentration of 2.0 micrograms/ml for all nine isolates tested.