Mycobacterium tuberculosis - TB

Summary

This video provides an informative and concise overview of Mycobacterium Tuberculosis (TB). It explains the characteristics of TB, including its unique cell wall structure with mycolic acid, its acid-fast properties, and its ability to infect macrophages. The video also covers the pathophysiology, including granuloma formation, Ghon focus, and reactivation in immunocompromised individuals. It touches on diagnostics, including skin tests and the Quantiferon-TB test, as well as treatment regimens like RIPE. Preventive measures such as better housing, nutrition, and the BCG vaccine are also discussed, along with the importance of proper TB control in healthcare settings.

Takeaways

• 😀 Mycobacterium tuberculosis (TB) is an obligate aerobic bacillus that primarily infects the lungs and requires oxygen to grow.
• 😀 TB has a unique lipid-rich cell wall containing mycolic acid, which makes it acid-fast and resistant to Gram staining.
• 😀 Ziehl-Neelson stain is used to detect Mycobacterium tuberculosis due to its acid-fast nature.
• 😀 TB infects macrophages, and its virulence factors, such as 'cord factor' and sulfatides, help it evade immune responses and survive inside cells.
• 😀 The primary infection of TB usually occurs in the lower or middle lobes of the lungs, while reactivated TB prefers the upper lobes due to a high V/Q ratio.
• 😀 Caseating necrosis, or cheese-like necrosis, is a hallmark of TB infection and leads to the formation of granulomas known as Ghon focus.
• 😀 TB can spread to nearby hilar lymph nodes, forming the Ghon complex, and may calcify into the Ranke complex over time.
• 😀 Latent TB occurs when the immune system contains the bacteria, but they remain dormant; it can reactivate if the immune system is compromised, such as in HIV.
• 😀 TB can spread beyond the lungs through the lymphatics and bloodstream, leading to conditions like miliary TB and Potts disease (spinal TB).
• 😀 The diagnosis of TB involves skin tests, chest X-rays, and sputum samples, with treatment involving a multi-drug regimen (RIPE: rifampin, isoniazid, pyrazinamide, ethambutol).

Q & A

What is the main characteristic of mycobacteria that gives it its name?

Mycobacteria are named for the presence of mycolic acid in their cell wall, which is a lipid-rich substance. This gives them unique properties, such as resistance to decolorization after staining.

Why are mycobacteria referred to as 'acid-fast'?

Mycobacteria are called 'acid-fast' because they resist decolorization by acid after being stained with carbolfuchsin. This property makes them poorly susceptible to gram staining.

Which stain is used to detect Mycobacterium tuberculosis (TB)?

The Ziehl-Neelsen stain is used to detect Mycobacterium tuberculosis, particularly due to its acid-fast properties.

What is the preferred site of primary TB infection in the lungs?

Primary TB infection typically occurs in the lower or middle lobes of the lungs, where there is less oxygen compared to the upper lobes.

What does it mean that TB is a facultative intracellular organism?

Being a facultative intracellular organism means that TB can live inside host cells, such as macrophages, when necessary, helping it evade the immune system.

What are some key virulence factors of Mycobacterium tuberculosis?

Key virulence factors of TB include Trehalose dimycolate (cord factor), sulfatides (which prevent phagosome-lysosome fusion), and catalase-peroxidase (which allows resistance to host cell oxidation).

How does TB evade the immune response after it infects macrophages?

TB evades the immune response by preventing the fusion of phagosomes with lysosomes in macrophages, allowing it to survive and replicate inside the cells.

What is the significance of a Ghon focus in TB infection?

A Ghon focus is an area of caseating necrosis (cheese-like tissue death) that forms as the immune system attempts to contain TB in the lungs. It typically forms in the mid to lower lobes of the lungs.

What happens when latent TB becomes reactivated?

When latent TB becomes reactivated, it starts to spread, often resulting in cavitation in the lungs, leading to more widespread infection. This can cause disseminated TB and potentially affect other organs.

What is the role of the BCG vaccine in TB prevention?

The BCG vaccine, made from a strain of Mycobacterium bovis, is used primarily in countries with high TB incidence. It provides partial resistance to TB but is not widely used in the U.S. due to its variable effectiveness.