Pathology of Tuberculosis (TB)
Mycobacterium tuberculosis is a highly contagious, airborne, rod-shaped organism (bacillus) that thrives on oxygen, grows slowly, and possesses a "waxy" cell wall. The cell wall's structure and function are not well understood but appear to allow the bacteria to survive within immune cells called macrophages (specialized cells that destroy bacteria and viruses). It also provides the organism with a resistant barrier to many common drugs.
M. tuberculosis is difficult to study in the laboratory. Slow growth makes culturing a lengthy process and colony formation can take several weeks. Also, TB bacilli form clumps, making them difficult to work with and count. Because tuberculosis is a dangerous airborne pathogen, study requires special safety equipment.
The bacteria's primary host is the human and infection spreads through direct person-to-person contact. When an infected person talks, coughs, sings, or spits, tiny aerosolized droplets containing bacteria are released into the air and inhaled by uninfected persons. Viable bacteria can remain in the air for a long time.
Once the bacteria are inhaled, they are engulfed by macrophages (white blood cells) that are present in the alveoli (the air sacs of the lungs). The bacteria replicate within the macrophages for 2 to 3 weeks before spreading throughout the body. In 95% of cases, the macrophages throughout the body are able to contain the bacteria and no apparent disease is noted. However, the bacteria are not completely destroyed and can remain dormant for years.
Granulomas prevent spread of infection by confining bacteria within a compact collection of several types of immune cells and activated macrophages, some of which fuse together. These cells work in various but specific ways to isolate, inhibit the replication of, and destroy the bacteria. At the center of this aggregate, toxic substances released by some of the immune cells create an unfavorable environment for the bacteria and most of them die.
The center has a soft, dry, crumbly cheese-like appearance and the granuloma is described as caseated (ka'-see-a'-ted; from the Latin word for cheese, caseus). Granulomas then become dormant and are sealed off by scar tissue. If any bacilli survive, they may reactivate years later.
What triggers reactivation is not well understood. Five years or more after infection, the bacteria can activate some immune cells to release a substance that renders host tissue cells sensitive to killing. Other immune cells are activated to release substances that liquefy the bacteria-containing center of the granuloma.
When the granuloma and surrounding tissue erode, the liquefied material is discharged into an airway and a cavity (enlarged air space) forms in the lung. Oxygen and carbon dioxide then freely enter the space, and bacilli replicate in enormous numbers, thriving in this now highly favorable environment. Bacilli spread through air passages from cavities to other parts of the lung and larynx. Swallowed sputum may cause lesions in the gastrointestinal (GI) tract (i.e., the alimentary tract or digestive tract).