- Mycobacterium tuberculosis
is an acid-fast, rod-shaped bacterium approximately
0.4µm by 3.0µm. Acid-fast refers to the fact that because of its unique
cell wall, when stained by the acid-fast procedure it will resist decolorization
with acid-alcohol and stain red,
the color of the initial stain, carbol fuchsin. With the exception of a very
few other acid-fast bacteria such as Nocardia, all other bacteria will
be decolorized and stain blue, the color of the methylene blue counterstain.
- It is an obligate aerobe (def).
- Because of its fastidious (def)
nature and a relatively impermeable cell wall, M. tuberculosis is
slow growing, dividing only every 12-24 hours.
- The cell wall also renders it
resistant to many disinfectants and antibiotics.
- Found in infected humans.
- Inhalation of aerosolized (def)
infectious particles from close person-to-person contact.
- An estimated one-third of the
world's population, approximately 1.9 billion people, are thought to be infected
with M. tuberculosis.
- It is estimated by the World Health
Organization that worldwide, there are over 8,800,000 million new cases of
tuberculosis each year and nearly 3,000,000 deaths.
- The disease is most prevalent
in sub-Saharan Africa, Southeast Asia, and Eastern Europe.
- There are approximately 20,000
newcases of tuberculosis a year in the U.S.
- In the U.S. tuberculosis is most
prevalent in immigrants, drug or alcohol abusers, the homeless, and immunocompromised
- Usually restricted to the lungs
in immunocompetent patients but may spread by way of the blood to any organ
in immunosuppressed patients.
- Around 5% of people exposed to
M. tuberculosis progress to having active disease within 2 years;
an additional 5%-10% develop disease sometime later in life.
- Progression from exposure to M.
tuberculosis and progression to active pulmonary disease is a function
of the number of bacteria entering the body and the competence of the person's
- Effectiveness of elimination of
the bacteria is partly related to the size of the focus of infection.
- Typical symptoms are malaise (def),
weight loss, cough, and night sweats. Sputum can range from scant to bloody
and purulent (def).
- When M. tuberculosis
enters the lungs it penetrates unactivated alveolar macrophages (def)
and and inhibits the acidification of phagosomes (def)
needed for lysosomal enzymes to kill the bacteria. M. tuberculosis
replicates freely in these alveolar macrophages eventually destroying them
and spreading to other cells.
- Most disease symptoms and damage
to the body is a result of immune responses to the bacterium. Eventually activated
engulf and kill the bacteria while cytotoxic T-lymphocytes (def)
kill M. tuberculosis-infected cells. If the bacterial load is small
at this time the bacteria are destroyed with minimal tissue damage. However,
if the bacterial load is high, production of inflammatory cytokines, activation
of the complement pathways, and the hydrolytic enzymes and toxic oxygen radicals
produced by macrophages lead to considerable tissue death.
- If the intial foci of infection
are small and the localized accumulations of activated macrophages (called
are less than 3 millimeters, the activated macrophages usually contain and
kill the M. tuberculosis. However, when larger granuloma develop
in areas with more tissue necrosis (def),
they become encased in fibrin (def)
and the bacteria are protected from macrophage killing. In this state the
M. tuberculosis can remain dormant for years and be reactivated if
immune defenses weaken as a result of aging, immunosuppressive diseases, or
immunosuppressive treatments. The
formation of granuloma is actually the result of cell-mediated immune responses
attempting to wall-off and localize infections that the body cannot effectively
remove with macrophages.
For a more detailed article on tuberculosis,
by Thomas Herchline, MD, Associate Professor, Department of Internal Medicine,
Division of Infectious Disease, Wright State University and Judith
K Amorosa, MD, FACR, Clinical Professor and Program Director, Department of
Radiology, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson
Medical School; Consulting Staff, Department of Radiology, Robert Wood Johnson
Kaiser's Microbiology Home Page
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Updated: January 6, 2005
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