Isoniazid susceptibility as a criterion for the differentiation of mycobacterial species from other mycolic acid-containing taxa

125 strains of mainly fast-growing mycobacteria and representatives of related taxa were screened for susceptibility to 50, 100, and 200 μg of isoniazid per ml. Over 80% of the mycobacterial strains were susceptible to 200 μg of isoniazid per ml, with the resistant ones restricted to the species Mycobacterium album, M. aurum, M. chelonei, M. farcinogenes, and M. fortuitum. With a single exception, representatives of the taxa Bacterionema, Corynebacterium, Micromonospora, Nocardia, Rhodococcus, Streptomyces, and Gordona aurantiaca were resistant to the concentrations of isoniazid tested. The susceptibility of most fast-growing mycobacteria to high concentrations of isoniazid seems to provide the basis of a useful test to distinguish the majority of these organisms from related actinomycete and coryneform bacteria.