Spectrum of drugs against atypical mycobacteria: how valid is the current practice of drug susceptibility testing and the choice of drugs?

The in vitro activity of 13 drugs against 552 clinical isolates of atypical mycobacteria representing 12 species was performed in 7H11 agar medium at the National Reference Laboratory for Mycobacteria, using the 1% proportion method. All the species tested were resistant to isoniazid and pyrazinamide. In general, clofazimine and D-cycloserine showed the widest spectrum of activity except in the case of Mycobactrium fortuitum and M. chelonei which were resistant to both drugs, and the M. szulgai and M. terrae complex which was resistant to D-cycloserine. The next broad-spectrum drug was ethionamide, followed by ansamycin, rifampin, capreomycin, kanamycin, streptomycin and ethambutol. Among the fluoroquinolones, both ciprofloxacin and ofloxacin were active against M. xenopi, M. gordonae and M. fortuitum whereas M. kansasii and M. gastri were sensitive to ofloxacin only. When the species were listed in respect of the number of drugs to which they were susceptible (less than 10% of resistant strains), they were classified as follows; 7/13 drugs for M. kansasii, M. gastri and M. xenopi; 6/13 for M. gordonae; 5/13 for M. marinum; 3/13 for M. szulgai; 2/13 for M. fortuitum; 1/13 for the M. avium, M. scrofulaceum, M. simiae, and M. terrae complex, and none of the 13 in the case of M. chelonei. These results are discussed in relation to the multiple drug resistance of atypical mycobacteria. We conclude that the critical concentrations of drugs established for M. tuberculosis are not appropriate for atypical mycobacteria.