Chapter 270

The nontuberculous mycobacteria (NTM) have been collectively identified by a variety of terms, including mycobacteria other than tuberculosis, atypical, nonpathogenic, unclassified, and environmental or opportunistic mycobacteria. Although grouping these organisms can be helpful, classification based on specific etiologic agent is preferable because this has implications for the predisposing factors, usual clinical course, diagnosis, and appropriate medical and surgical management of the infection.

Mycobacteria are true bacteria. They are nonmotile, nonspore-forming, slender pleomorphic rods. Their cell walls have a complex structure that includes a variety of proteins, carbohydrates, and lipids. Studies using high-pressure liquid chromatography (HPLC) reveal a variable species-related distribution of mycolic acids, each species having a distinct mycolic acid fingerprint that can be used for identification.

### Epidemiology

More than 60 species of Mycobacterium have been described, of which about half are pathogenic in humans. The most commonly encountered are Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium scrofulaceum, which are classified together as the M avium complex (MAC).

The direct detection of nontuberculous mycobacteria (NTM) is similar to that for Mycobacterium tuberculosis. All NTM are acid fast but these are visualized in fluid and tissue samples less than 50% of the time. Although even a single organism visualized on an entire slide is suspicious, false-positive results can be caused by contamination of stain solutions, tap water, distilled water, delivery tubes, or immersion oil. Direct detection of the various NTM by nucleic acid amplification is advancing, but appropriate primers and reagents are not yet commercially available for many species.

Methods used for the isolation of M tuberculosis from clinical samples are also useful for the isolation of NTM. All mycobacteria are obligate aerobes that grow best in the presence of 5% to 10% CO2. Isolation on solid media of slow-growing NTM takes 2 to 6 weeks. Only the rapid growers (Mycobacterium fortuitum, Mycobacterium chelonei, and Mycobacterium abscessus) form visible colonies in less than 10 days. Use of liquid media systems usually leads to isolation of any species of NTM within 14 days. Some newly recognized species of mycobacteria cannot be cultivated but can be detected by nucleic acid amplification. Many clinical laboratories now use high-pressure liquid chromatography (HPLC) analysis to speciate these organisms.

Determining the species of NTM causing infection is crucial to directing chemotherapy. Although drug susceptibility testing of MAC isolates is not predictive of clinical response and does not contribute significantly to care of the patient, susceptibility testing for the rapid-growing mycobacteria can be informative. For these mycobacteria, susceptibility testing to antibiotics such as amikacin, cefoxitin, doxycycline, sulfonamides, linezolid, and the macrolides may be particularly helpful.

Transmission of NTM to humans occurs from environmental sources, including soil, water, dust, and aerosols. NTM have been isolated from as many as 80% of soil samples, and certain strains of MAC are found in fresh and brackish waters in warmer climates. Other mycobacteria ...

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