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 distinguishing between tuberculosis and non-tuberculous mycobacteria. Interestingly, there are more than 170 different species of NTM. Fewer labs are equipped to determine exactly which NTM organism it might be and its susceptibility to antibiotics. This is important to determine the best treatment for NTM.
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 species that may require treatment include: M. avium complex, M. kansasii, M. abscessus, M. chelonae, M. fortuitum, M. terrae, M. xenopi, M. simiae, M. szulgai and M. malmoense. Among the NTM, there are three species that predominantly involve the skin: M. leprae, M. ulcerans and M. marinum.