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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, 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.
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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).
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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.
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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.
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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.
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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 ...