Mycobacterium tuberculosis Morphology, Characteristics, Acid fast Stain and Culture Media

Photo of Mycobacterium tuberculosis Morphology, Characteristics, Acid fast Stain and Culture Media

What is Mycobacterium tuberculosis?

Mycobacterium Tuberculosis is an infectious microorganism that causes Tuberculosis in humans. It is a type of Mycobacteria which include 2 species that cause diseases in humans Mycobacterium tuberculosis and Mycobacterium leprae (which causes Leprosy

Mycobacterium Tuberculosis Epidemiology

Mycobacterium Tuberculosis is the second infectious diseases causing morbidity and mortality in the developing world and 1/3 of the worlds population is infected with Mycobacterium Tuberculosis with 30 million people have active tuberculosis. There are about 9 million new cases every year and 3 million people die of Mycobacterium Tuberculosis disease yearly.

Mycobacterium tuberculosis Morphology/Structure

The structure of Mycobacterium Tuberculosis when viewed under the microscope shows slender rods (this why they are called Bacilli rods). This bacterium is difficult to staining but once stained, it becomes resistant to decolorization by diluted mineral acids this is the reason why Mycobacterium tuberculosis is called Acid Fast bacilli (AFB).

Characteristics of Mycobacterium tuberculosis

Mycobacterium tuberculosis belongs to genus Mycobacteria (this is the category M. Tuberculosis in taxonomy). The following characteristics are found in any organism found in this genus.

  1. They are Aerobic microorganisms: this means that they require oxygen to survive and cause infection this is the reason why Mycobacterium tuberculosis affects the lungs the most because it gets all the needed oxygen in it.
  2. They are Non-motile: they cannot move because they have no structures for movement such as cilia.
  3. They are Non-spore forming: they do not form spores
  4. They are Non-capsulated: they have no capsules covering their cell wall.
  5. They grow slowly this is the reason why it takes time to show signs and symptoms of mycobacterium tuberculosis and it also takes time to treat. When culturing mycobacterium tuberculosis, the tubercle colonies appear after 2 weeks or at 6-8 weeks
  6. They are Obligate parasites that depend on others to survive
  7. They are Opportunist pathogens that can cause serious infections when the immune system is suppressed by the presence of disease or drugs (in immunosuppression)
Incidence, Prevalence and Deaths due to Mycobacterium tuberculosis infection worldwide: showing Africa, America, Europe and other countries
Incidence, Prevalence and Deaths due to Mycobacterium tuberculosis infection worldwide: showing Africa, America, Europe and other countries


Resistance of Mycobacterium tuberculosis to heat and chemicals

  1. It is relatively resistant to heat but becomes destroyed when heated to 60 degrees Celsius for 20 minutes.
  2. It can survive in sputum for about 20 to 30 hours
  3. Mycobacteria are relatively resistant to disinfectants and can survive after exposure to any of these chemicals: 5% Phenol, 15% Sulphuric acid, 3% Nitric acid, 5% Oxalic acid and 4% sodium hydroxide (NaOH)

Staining of Mycobacterium tuberculosis in the Laboratory

  1. The procedure for staining mycobacterium tuberculosis involves taking of sample such as a smear of sputum and covering it with a red stain known as carbolfuchsin.
  2. This is then heated in order to help the penetration of the added dye.
  3. A mixture of Acid and alcohol in the ratio: 95% of ethanol and 3% of Hydrochloric acid (HCl) is then poured over the heated sputum smear.
  4. A counter-stain of methylene blue is then applied.
  5. When this is done to other bacteria that are not acid fast, their cell walls normally dissolve and the red color of the stain washes off and they also take the blue color of the counter stain (methylene blue).
Chest X-ray showing Chronic Mycobacterium tuberculosis disease
Chest X-ray showing Chronic Mycobacterium tuberculosis disease


Types of Culture Media for growing Mycobacterium tuberculosis

  1. Solid media
  2. Liquid media

Culture Media is like a broth containing all the needed ingredients for the appropriate growth of specific microorganisms. Because of this, there are different types depending on the ingredient added to it.

Types of Solid Culture Media used for culturing Mycobacterium tuberculosis

  1. Egg containing Solid culture media: this include: Lowenstein-Jensen MediumPetragnini medium and Dorset medium
  2. Blood containing Solid culture media: example is the Tarshis medium
  3. Serum containing solid culture media: example is the Loefflers serum slope
  4. Potato containing culture media: example is the Pawlowskys medium

Types of Liquid Culture Media used for culturing Mycobacterium tuberculosis

  1. Dubos Media
  2. Middlebrooks Media
  3. Proskauer & Becks Media
  4. Sulas Media
  5. Sauton Media

Culturing Mycobacterium tuberculosis can therefore be carried using any of these media: either the liquid media or solid media.

Why Mycobacterium Tuberculosis is said to be Acid Fast Bacilli

Bacteria that are not acid fast when stained absorb the blue color of counter stain but this does not occur in acid fast bacteria. Reverse is the case for mycobacteria – because their cell walls do not dissolve and the color of the red stain does not wash off too; therefore, acid-fast organisms resist decolorization with acid alcohol – holding fast to their red stain (hence the name Acid Fast Bacilli).

Mycobacterium tuberculosis transmission

  1. Droplet infection: this occurs through person to person transmission by inhalation of aerosols when an infected person coughs, speaks, signs or laughs
  2. Contamination of skin abrasion this occurs mostly in Laboratory workers

Symptoms and Signs of Mycobacterium tuberculosis infection

  1. Low grade fever
  2. Cough: may be productive of blood stained sputum
  3. Drenching night sweats
  4. Weight loss
  5. Anemia
  6. Fatigue
  7. Hemoptysis (coughing of blood)
  8. Dyspnea (difficulty in breathing)
  9. Other symptoms are dependent on the part of the body affected outside of the lungs such as in Extrapulmonary tuberculosis

Laboratory Diagnosis of Mycobacterium tuberculosis infection

  1. Culture of needed specimen
  2. Tuberculin skin test (mantoux test)
  3. Chest X ray for pulmonary tuberculosis
  4. Polymerase chain reaction (PCR) test

Types of specimen that can be collected for Culturing Mycobacteria tuberculosis

  1. Sputum
  2. Urine
  3. Body fluids such as pleural fluid, peritoneal fluid, synovial fluid etc
  4. Gastric lavage
  5. Blood
  6. Tissue biopsy

The type of specimen collected is based on the organ or part of the body affected by tuberculosis.

Mycobacterium tuberculosis Treatment

The treatment guideline for infection by mycobacterium tuberculosis is the same as for pulmonary tuberculosis. It involves the use of Rifampicin, Isoniazid, Ethambutol and Pyrazinamide. Treatment is based on the category of the patient using the W.H.O classification; for more on pathogenesis and treatment of Tuberculosis please read about Pulmonary Tuberculosis.

Mycobacterium tuberculosis infection prevention

Prevention is done by vaccination of children at birth using the BCG vaccine. This vaccine protects against Tuberculosis and even when infected, it will be milder and it will prevent against having skeletal TB, meningeal and miliary Tuberculosis forms.


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