Under the microscope, nontuberculous mycobacteria and tuberculosis appear quite similar. Careful lab studies must be performed to tell them apart. Most labs are capable of carrying the testing process far enough to determine whether they are dealing with an NTM. Fewer labs are equipped to determine exactly which organism it might be and what its susceptibility is to drugs.
The importance of identifying the exact organism can be illustrated with two of the organisms, Mycobacterium gordonae and Mycobacterium scrofulaceum. These two are very similar and react the same way in many lab tests. However, they react in different ways in the human body. One organism causes disease; the other organism does not cause disease. In this case, if the organism turns out to be M. gordonae, treatment is seldom indicated. M. gordonae is often a lab contaminant and not a cause of human disease. In fact, M. gordonae is found in water supplies so often that it is nicknamed "the tap water bacillus". M. scrofulaceum, on the other hand, is known to cause disease and may require specific forms of treatment.
Other NTM infections that require treatment include: M. avium, M. intracellulare M. kansasii, M. abcessus, M. chelonae, M. fortuitum, M. terrae, M. xenopi, and M. simiae, M. szulgai, and M. malmoense. Among the NTM, there are three species which predominantly involve the skin: M. leprae, M. ulcerans, and M. marinum.