Microbiology
Summary
Mycobacterium tuberculosis is an acid-fast, obligate aerobic bacterium that typically infects the lungs and is spread from human to human through respiratory droplets. This bacterium cultures very slowly, often requiring anywhere from 2 up to 6 weeks for growth. In terms of its growth medium, it requires a special component called the Lšwenstein-Jensen medium, which is used for the isolation and cultivation of mycobacteria. The Mycobacterium cell wall contains cord factor, glycolipids involved in bacterial clumping into a serpentine-like formation. Another virulence factor of M. tuberculosis are sulfatides, which allow the bacterium to survive inside cells by preventing phagolysosome fusion.
If the immune system does not promptly eliminate M. tuberculosis following infection, the bacterium tends to inhabit and reproduce within macrophages, ultimately leading to the development of tuberculosis (TB). The infection pathway of TB presents three outcomes: a healed latent infection, a severe systemic infection (miliary TB), or a previously latent infection that reactivates later in life. Primary infection typically affects the lower or middle lobes of the lungs, but should it reactivate, it often affects the upper lobes. Three classic symptoms of reactivation are cough, night sweats, and hemoptysis (bloody cough). Other complications include Pott disease (tuberculosis that primarily affects the spine, leading to vertebral destruction and deformities), or infections of the central nervous system (CNS), causing meningitis or a tuberculoma.
Treatment for TB involves a combination of drugs including rifampin, isoniazid, pyrazinamide, and ethambutol (RIPE). In the case of latent TB, Rifampin and isoniazid are used for prophylaxis.
Lesson Outline
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FAQs
Mycolic acids are unique, long-chain fatty acids that are found in the cell walls of M. tuberculosis. These mycolic acids contribute to the organism's resistance to disinfectants and antibiotics, as well as to its ability to survive within macrophages. They are also responsible for the acid-fast characteristic of the bacteria, a crucial factor in laboratory identification.
TB is usually transmitted through small airborne droplets that are released when an individual with active tuberculosis coughs, sneezes, or talks. These minute droplets contain the Mycobacterium tuberculosis bacteria, which can be breathed in by people nearby, resulting in potential infection.
Cord factor and sulfatides are both important virulence factors of M. tuberculosis. Cord factor, a glycolipid, can inhibit macrophage activation, promote granuloma formation, and cause serpentine cording in culture, which is unique to pathogenic mycobacteria. On the other hand, sulfatides impede the fusion of phagosomes (which engulf the bacteria) with lysosomes (which contain enzymes to destroy pathogens), enabling the bacterium to evade the host's immune response.
Tuberculosis can persist in a dormant or latent state in the human body for many years. Reactivation of TB typically occurs when the person's immune system becomes weakened, allowing the dormant bacteria to multiply and cause disease. TNF-alpha, a potent immune regulatory cytokine, plays an important role in controlling the latent TB infection. Inhibition or deficiency of TNF-alpha can impair containment of the bacteria and promote reactivation of latent TB.
Lšwenstein-Jensen medium is a special egg-based culture media used to grow Mycobacterium tuberculosis in the lab. It allows for necessary biochemical testing and drug susceptibility testing. In terms of management, RIPE (rifampin, isoniazid, pyrazinamide, ethambutol) treatment is the standard first-line therapy for TB. It involves administration of these four antimicrobial drugs, which work together to eliminate the bacteria and avoid the development of drug resistance.
