INTRODUCTION:-

Mycobacteria (Myces meaning funges ) are slender bacilli that show breathing filamentous forms sometimes, resembling fungal mycelium.

CLASSIFICATION:-

 Mycobacterium are classified as-

1. CULTIVABLE

i. Typical Tubercle Baciui(Mammalian)
  • M.tuberculosis
  • M. Bovis
  • M. Microti
  • M. africanum
ii. Atypical Mycobacteria
  • Photochromogens
  • Scotochromogens
  • Non-photochromogens
  • Rapid growers

iii. Mycobacteria Causing Skin Ulcers
  • M. Ulcerans
  • M. Balnei


iv. Saprophytic Mycobacteria
  • M. Smegmation
  • M. Butyricum
  • M. Stercoris
  • M. Phlei

2. NON-CULTIVABLE

  • M.leprae 

MORPHOLOGY:-

SHAPE:- It is a slender, straight or slightly curved bacillus with rounded ends.
SIZE:- 1-4um*0.2-0.8um.
NATURE:- These bacilli are acid-fast, non-sporing,non-capsulated and non-motile.
STAINING:- They are Gram-positive but difficult to stain with gram stain. The stain used is Ziehl-Neelsen. They appear as bright red.

CULTURE:

  • M. tuberculosis is an obligate aerobe.
  • Most commonly used culture - Lowenstein Jenson (LJ) Medium.
  • The optimum temperature required for growth is 37°C.
  • Optimum pH:- 6.4-7.0.
  • Incubation period:- The bacilli grow slowly and colonies appear about only two weeks.
  • Appearance:- Colonies of M. Tuberculosis are dry Rough, buff-colored, raised with wrinkle surface.
  • Growth:-  Eugenic (luxuriant) growth.


RESISTANCE:-

  • Mycobacteria are killed at 60 °c in 15-20 mins.
  • They are sensitive to the UV rays and sunlight.
  • Bacilli in culture may be killed by direct sunlight exposure for 2 hours, but Bacilli present in sputum may remain alive for 20-30 hours.
  • They are relatively resistant to chemical disinfectant.
  • They are destroyed by tincture of iodine in 5 min and by 80 % ethanol in 2- 10 min.

PATHOGENESIS:-

  • The infection is only acquired by inhalation of infected droplets coughs or sneezed into the air by a patient with pulmonary tuberculosis.
  • Tubercle bacilli are inflicted by macrophages but they survive and multiply in macrophages. These lyse the host all infect other macrophages of lungs. 

Human Tuberculosis:- 


1. Primary Tuberculosis:-

Inhaled Tubercle bacilli are engulfed by alveolar macrophages in which they replicate to form a lesson called GHON FOCUS. 

2. Secondary Tuberculosis:-

It is caused by reactivation of the primary lesion or by exogenous infection. 

TUBERCULIN TEST / MANTOUX TEST:- 

Principle:-

It is delayed or type IV hypersensitivity reaction. 

Reagents:-


A) Old tuberculin (OT):- It was originally described by Robert Koch. It is a crude preparation of the 6-8 week culture filter of tubercle bacilli, concentrated by evaporation on a heated water bath. 

B) Purified Protein Derivative (PPD):- It is the purified preparation of the active tuberculoprotien prepared by growing M.tuberculosis in a semi-synthetic medium. 

Medium :- 

Mantaux Test :-

0.1 mm of PPD containing 5TU is injected intradermally into the flexor aspect of the forearm. A PPD the dose of 1TU is used when extreme hypersensitivity is suspected.

Heaf Test:-

A drop of undiluted PPD is spread on the area of skin. The multiple punctured apparatus is pressed against this area of skin.

Result:-

In  Monteux test the site of Infection is examine after 48-72 hrs of interpreted as- 

Positive Test:-

Induration(local edema) of 10mm diameter or more surrounded by erythema at the site of inoculation.
It only confirms past Infection with T. Bacilli but doesn't indicate the presence of active stage of the disease. 

False Negative:-

The conditions are - 
Miliary Tuberculosis.
Infection of measles, Hodgkin's disease, malnutrition, administration of immunosuppressive agents. 

False-positive:-

In these types of mycobacteria such as atypical mycobacteria.

Uses:-

  • To measure the prevalence of Infection in a community. 
  • To diagnose active Infection in young children. 
  • It is used as an indicator of the success of BCG vaccination. 

LAB DIAGNOSIS:-

 

1. Specimen:- from the site of infection. 
  • Pulmonary tubule 
  • Meningitis
  • Renal Tuberculosis
  • Bone and joint tuberculosis .
  • Tissue.

2. Direct Microscopy 
3. Culture Method 
4. Serology- detection of antimycobacterial antibodies.
5. Chromatographic method.