Mycobacteria

Published on 18/02/2015 by admin

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Last modified 18/02/2015

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Mycobacteria

I Shared Mycobacterial Properties

• Mycobacteria are slow-growing, aerobic, facultative intracellular rods with a lipid-rich cell wall that makes them acid fast.

Mycobacteria are acid-fast bacilli.

Mycobacterial cell wall contains unusual lipids that protect the bacteria from being killed after phagocytosis.

Mycobacteria are slow growing and cultured on Lowenstein-Jensen medium.

A Cell wall components

• The unusual composition of the mycobacterial cell wall is depicted in Figure 16-1.

16-1 Schematic depiction of the mycobacterial cell wall, which consists of three major layers.

B Mycobacterial diseases (Table 16-1)

TABLE 16-1

Mycobacterial Diseases

• Most mycobacteria cause chronic diseases that often exhibit a prolonged latent period as well as periods of remission alternating with active disease manifestations.

II Mycobacterium tuberculosis

Trigger words:

M. tuberculosis: acid fast, caseation, Ghon complexes, granuloma, isoniazid, Löwenstein-Jensen medium, Mantoux reaction, opportunistic disease, PPD

A Identification (Box 16-1)

1. Microscopic detection of acid-fast rods in sputum or biopsy specimen

2. Isolation by culturing on egg-based Löwenstein-Jensen medium or on special broth media

• Slow growth of M. tuberculosis requires 3 to 6 weeks of incubation.

M. tuberculosis is slow growing and uses Löwenstein-Jensen medium.

3. Serologic tests and DNA probes

4. Tuberculin skin test

• Intradermal injection of purified protein derivative (PPD) from cell wall induces a delayed-type hypersensitivity (DTH) response in those who have been previously exposed to M. tuberculosis or vaccinated.

Infection identified by DTH response to PPD or interferon-γ production in blood test

• Positive reaction is indicated by an area of induration (>15 mm for healthy adults) 48 to 72 hours after PPD injection.

a. Tuberculin skin test is a classic example of a type IV hypersensitivity reaction (DTH).

b. Skin test reactivity usually develops 3 to 4 weeks after infection.

c. False-negative results may occur in those with very recent infection, anergic individuals (especially human immunodeficiency virus [HIV]-infected patients), and older people in whom the DTH response has waned.

d. False-positive results occur in individuals vaccinated with bacille Calmette-Guérin (BCG), the antituberculosis vaccine used in Europe and other countries.

B Transmission and incidence

1. Humans are the only natural reservoir of M. tuberculosis

2. Spread of tubercle bacilli through respiratory droplets is promoted by crowded conditions and coughing.

3. Young children, elderly people, and immunocompromised individuals have the highest risk for developing active tuberculosis (TB).

C Pathogenesis

• Infection with M. tuberculosis may involve any organ, but the lungs are the initial and most common sites affected.

1. Inhaled mycobacteria are engulfed by alveolar macrophages and replicate freely in these cells.

• Cell wall components prevent bacterial destruction in macrophage lysosomes.

• Intracellular growth protects mycobacteria from antibody-mediated elimination.

• Other macrophages are attracted to the site and destroy the infected cells, releasing mycobacteria that can spread through the bloodstream.

Virulence factors: intracellular growth, dormant presence in macrophages, lipid-containing cell wall

2. Sequence of formation of a tuberculous granuloma (type IV hypersensitivity reaction)

• Tubercle bacilli are phagocytosed by alveolar macrophages.

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