[Differentiation between Mycobacterium malmoense and Mycobacterium nonchromogenicum by ethambutol susceptibility]

Mycobacterium malmoense strains are resistant to 1 microgram/ml ethambutol in Ogawa egg medium, whereas Mycobacterium nonchromogenicum strains are susceptible to this concentration. Therefore, these two Group III nonphotochromogens are differentiated clearly from each other by the susceptibility test for ethambutol 1 microgram/ml. Ninety-five % of Mycobacterium avium complex strains are resistant to this concentration, while M. nonchromogenicum strains are susceptible. Therefore, the test serves for differentiating M. nonchromogenicum from M. avium complex. Previously, Tsukamura (Kekkaku 45: 237, 1970) reported that pathogenic and nonpathogenic mycobacteria of Group II and Group III are differentiated by the susceptibility test to ethambutol, 5 micrograms/ml. However, the use of Ogawa egg medium containing 1 microgram/ml ethambutol is superior to that of 5 micrograms/ml ethambutol for differentiating pathogenic and nonpathogenic ones of the Group III.