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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.
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Mycobacteria are true bacteria. They are nonmotile, non–spore-forming, weakly gram-positive slender pleomorphic rods. Their cell walls have a complex structure that includes a variety of proteins, carbohydrates, and lipids. Traditional species identification involved various biochemical tests that were time consuming and sometimes difficult to interpret. Studies using high-pressure liquid chromatography (HPLC) reveal a variable species-related distribution of mycolic acids, with each species having a distinct mycolic acid fingerprint that can be used for identification. However, genetic sequencing is used to identify species in research laboratories and is increasingly being used by reference clinical laboratories.
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PATHOGENESIS AND EPIDEMIOLOGY
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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 prevalence of infections caused by the so-called rapid growers—Mycobacterium fortuitum, Mycobacterium chelonae, and Mycobacterium abscessus—appears to be increasing, especially in patients with indwelling catheters and patients with immune compromise or cystic fibrosis (CF).
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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 have been isolated from natural water supplies and tap water. Although mycobacteria are frequently found in animals, particularly swine and poultry, there is little evidence to suggest direct animal-to-human transmission. There is no evidence that person-to-person transmission occurs, although concern has been raised that transmission of NTM can occur between patients with CF. Clusters and isolated cases of healthcare-associated disease due to NTM are being reported with increasing frequency. Most common are outbreaks caused by the rapid growers, which are associated with injectors, continuous ambulatory peritoneal dialysis, contaminated skin marking, and injection solutions and hemodialysis.
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The direct detection of NTM is similar to that for Mycobacterium tuberculosis. All NTM are acid fast, but they are visualized in stains of clinical 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.
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The true incidence and prevalence ...