Mycobacteria are slender, rod-like bacilli. They belong to the
order of Actinomycetales, microbes of the soil. The mycobacteria
have complex cell walls rich in lipids and waxes. These cell walls are
related to several distinctive features including their staining
properties and growth rates.
Although classified as gram-positive, the mycobacteria usually
do not take up gram stain and thus appear as “ghost” or
gram-neutral forms. Classically the mycobacteria have been stained
with the red dye, carbol fuchsin, enhanced by heating or alkalinity.
So stained, the mycobacteria are resistant to decolorizing with
the potent agent acid-alcohol. This has given rise to the designation “acid-fast
bacilli,” or AFB. Modern laboratories may use the fluorochrome
technique employing a dye, auramine-o, which fluoresces yellow when
excited by ultraviolet light.
Most species of mycobacteria are characterized by very slow growth
on culture media, replicating usually in the range of 18–24
h. Thus, laboratory isolation of mycobacteria usually requires 5–15 days
in liquid media and 15–30 days on solid media.
The most common human pathogens are Mycobacteriumtuberculosis and M
leprae (Hansen’s bacillus, which causes the cutaneous-neural
disease leprosy). However, there is an array of related microbes
found widely in the soil and water that cause lung and extrapulmonary disease
in humans. These latter organisms have been referred to variably
as atypical TB, mycobacteria other than TB (MOTT), nontuberculous
mycobacteria (NTM) or environmental mycobacteria (EM). In this chapter, “environmental
mycobacteria” will be used.
- • Tuberculosis organisms are spread from human
to human but are not exquisitely communicable.
- • Only 10% of people with normal immune
function develop active disease following a primary infection.
- • Primary tuberculosis is most often a nondescript
infiltrate with ipsilateral reactive hilar or mediastinal nodes.
- • Reactivation tuberculosis is characterized by respiratory
symptoms with upper lobe fibronodular infiltrates with or without
- • Tuberculosis is usually limited to the lungs, with
lymph nodes and pleura the only common sites of extrapulmonary spread.
- • Identification of acid-fast organisms by staining
or culture is diagnostic of disease.
Among the mycobacteria there is a group of closely related microbes
that is referred to as the “tuberculosis complex.” The
group includes M tuberculosis, M bovis, M
canetti, M africanum, and M microti. Although the microbes are
highly comparable by DNA analysis, they are generally distinguishable
in terms of natural reservoirs, human pathogenicity, and transmission
patterns. Thus, I believe it is appropriate clinically to preserve
the term “tuberculosis” or TB for diseases caused
by M tuberculosis. Disease caused by
other mycobacteria should, I believe, be referred to as “mycobacteriosis” due to “M x.”
TB is arguably the most “successful” parasite
of humans. The World Health Organization (WHO) estimates that roughly
one-third of the world’s population or 2 billion persons
are infected with M tuberculosis. However,
the patterns ...