Use of the BACTEC system for drug susceptibility testing of Mycobacterium tuberculosis, M. kansasii, and M. avium...

A total of 199 Mycobacterium tuberculosis, 182 M. kansasii, and 32 M. avium complex isolates were tested for susceptibility to either three or 10 color-coded drugs in a four-phase study using the conventional 7H10 plate method and the BACTEC system. Agreement between the two test systems for M. tuberculosis isolates against isoniazid, ethambutol, and rifampin was 96%; it was 93.6% against all 10 drugs. Disagreement was highest for ethionamide and cycloserine. M. kansasii data were encouraging, with 88.9% drug agreement against 10 drugs, but M. avium complex agreement was only 51.6% against 10. Average time required for reportable results was 4.8 days for M. tuberculosis, 4.3 days for M. kansasii, and 3.4 days for M. avium complex. Growth for 3 days in an enrichment medium yielded suspensions of the organisms in a logarithmic growth phase and reduced problems of clumps of cells in inocula.