Mycobacterium Tuberculosis (MTB)

M. tuberculosis has an unusual, waxy coating on its cell surface primarily due to the presence of mycolic acid. This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear either Gram-negative or Gram-positive. Acid-fast stains such as Ziehl-Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, culture, and polymerase chain reaction.

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Important Facts

1. Mycobacterium tuberculosis is the causative agent.
2. It is estimated that 20 to 43 % of the world population is suffering from TB.
3. In the USA 15 million people are infected (Old statics).
4. TB occurs in :
   1. Poor community, considered to be the disease of poor people.
   2. Malnourished people.
   3. Homeless.
   4. Overcrowded community.
   5. Substandard housing.
5. Mode of spread: This is an airborne disease.
   1. Primary TB = Clinically and radiologically is silent.
   2. Latent TB = Do not have active disease and can not spread the disease to others.
   3. Active TB = 10% of the latent TB develop active TB when not given treatment.
   4. Progressive primary TB = 5 % of the primary active TB with sign and symptoms.
6. This is thought that 90% of the disease is a reactivation of latent TB.

Sign and Symptoms

1. The patient will have :
   1. Malaise.
   2. Anorexia.
   3. Weight loss.
   4. Fever.
   5. Night sweating.
   6. A chronic cough is the common presentation of pulmonary TB.
   7. Blood streaked sputum is common.
   8. The patient may have hemoptysis.
   9. Rarely patients are asymptomatic.
   10. In advanced disease:
       1. There may be clubbing of nails.
       2. Enlarged lymph nodes in the neck.
       3. The patient may develop a pleural effusion.

Laboratory diagnosis

1. Definite diagnosis depends upon the demonstration of T.Bacilli by:
   1. Culture.
   2. Culture on solid media needs 12 weeks.
   3. Culture on liquid media needs several days.
   4. PCR by DNA or RNA amplification method.
2. Sputum, three consecutive sample is recommended for:
   1. Fluorochrome staining with rhodamine-auramine.
   2. AFB stain or Ziehl-Neelsen stain.
   3. Early morning specimen is recommended.
3. Bronchoscopy is advised for bronchial washing in case of negative sputum.
4. Transbronchial lung biopsy increases the diagnostic yield.
5. Gastric aspiration. Early morning sample is an alternative to bronchoscopy.
6. Blood culture, 15 % of the case may give a positive culture to T.bacilli.
7. Sensitivity should be done once the culture is positive.
   1. Sensitivity should be done if the sputum culture is positive after the treatment for 2 months.
8. Needle biopsy of the pleura shows the granulomas in 60 % of the cases.
9. Pleural fluid cultures are positive in < 25 % of the cases.
10. Radiology,  X-ray chest shows small homogenous opacity.
11. Mantoux test or Tuberculin test (TT).
    1. TT will not distinguish between latent or active TB.
    2. 0.1 ml (5 tuberculin units) of PPD should be injected intradermally.
    3. The best site is the volar surface of the arm.
    4. Injected with 27 G needle.
    5. Read after 48 to 72 hours for induration (thickening of the injected area).
    6. It takes 2 to 10 weeks to develop an immune response to PPD after the infection.
12. Other specimens can also be used are:
    1. Urine. The first-morning clean catch is collected for three consecutive days.
    2. Stool. This should be collected in a clean sterile container.
    3. Blood. Lysed centrifuged blood is used for culture.
13. Niacin test. Mycobacterium produces Niacin. This can be tested by commercially available kits.

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