Table of Contents
- What is Mycobacterium tuberculosis?
- Characteristics of Mycobacterium tuberculosis
- Types of Culture Media for growing Mycobacterium tuberculosis
- Why Mycobacterium Tuberculosis is said to be Acid Fast Bacilli
- Mycobacterium tuberculosis symptoms
- Laboratory Diagnosis
- Specimen for Culturing
What is Mycobacterium tuberculosis?
Mycobacterium Tuberculosis is an infectious microorganism that causes Tuberculosis in humans. It is a type of Mycobacteria that includes 2 species that cause diseases in humans Mycobacterium tuberculosis and Mycobacterium leprae (which causes Leprosy).
Mycobacterium Tuberculosis is the second infectious disease-causing morbidity and mortality in the developing world and 1/3 of the world’s population is infected with it. There are about 9 million new cases every year and 3 million people die of Tuberculosis disease yearly.
The structure of Mycobacterium Tuberculosis when viewed under the microscope shows slender rods (this is 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 the genus Mycobacteria. The following characteristics are found in any organism found in this genus.
- 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.
- They are Non-motile: they cannot move because they have no structures for movement such as cilia.
- They are Non-spore forming: they do not form spores
- They are Non-capsulated: they have no capsules covering their cell wall.
- 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
- They are Obligate parasites that depend on others to survive
- They are Opportunist pathogens that can cause serious infections when the immune system is suppressed by the presence of disease or drugs (in immunosuppression)
Resistance to heat and chemicals
- It is relatively resistant to heat but becomes destroyed when heated to 60 degrees Celsius for 20 minutes.
- It can survive in sputum for about 20 to 30 hours
- 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 in the Laboratory
- The procedure for staining mycobacterium tuberculosis involves taking sample such as a smear of sputum and covering it with a red stain known as carbolfuchsin.
- This is then heated in order to help the penetration of the added dye.
- 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.
- A counter-stain of methylene blue is then applied.
- 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 counterstain (methylene blue).
Types of Culture Media for growing Mycobacterium tuberculosis
- Solid media
- 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.
Solid Culture Media
- Egg containing Solid culture media: this includes: Lowenstein-Jensen Medium, Petragnini medium, and Dorset medium
- Blood containing Solid culture media: an example is the Tarshis medium
- Serum containing solid culture media: an example is the Loeffler’s serum slope
- Potato containing culture media: an example is the Pawlowskys medium
Liquid Culture Media
- Dubos Media
- Middlebrooks Media
- Proskauer & Becks Media
- Sula Media
- Sauton 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. The 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).
- Droplet infection: this occurs through person to person transmission by inhalation of aerosols when an infected person coughs, speaks, sings or laughs
- Contamination of skin abrasion occurs mostly in Laboratory workers
Mycobacterium tuberculosis symptoms
- Low-grade fever
- Cough: may be productive of blood-stained sputum
- Drenching night sweats
- Weight loss
- Hemoptysis (coughing of blood)
- Dyspnea (difficulty in breathing)
- Other symptoms are dependent on the part of the body affected outside of the lungs such as in Extrapulmonary tuberculosis
- Culture of needed specimen
- Tuberculin skin test (mantoux test)
- Chest X-ray for pulmonary tuberculosis
- Polymerase chain reaction (PCR) test
Specimen for Culturing
- Body fluids such as pleural fluid, peritoneal fluid, synovial fluid, etc
- Gastric lavage
- Tissue biopsy
The type of specimen collected is based on the organ or part of the body affected by tuberculosis.
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.
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.
Dr. Brown is the founder of Jotscroll, he is a Medical Doctor, Entrepreneur, and author. Dr. Razi Brown holds a medical degree from the University of San Diego. He has invested in many startups and is currently working on his fifth book to be published in the upcoming year.