Tubercle

The characteristic lesion of tuberculosis is the formation of tubercle which is a small rounded nodule in the tissue . Microscopically , there is central area of necrosis or caseation of tubercle . This is surrounded by nodular collection of epitheloid cells and scattered langhan’s type of giant cells . the epitheloid cells are modified mononuclear cells with a clear cytoplasm and pale vesicular nucleus . A zone of lymphocytes is present around the epitheloid cells . The zone of lymphocytes is surrounded by a zone of fibrous tissue and granulation tissue 

Staining properties of Mycobacterium tuberculosis

Tubercle bacilli  staining is very difficult due to presence of fatty acid and alcoholic material . Special type of staining is needed for the detection of tubercle bacilli like Ziehl-Neelsen technique . Due to presence of higher group of alcohol “Mykol” surrounding the body of the tubercle bacilli  , the organism when stained is resistant to the decolourising action of acdids and alcohol . for this property  is called acid fast organism .

 Properties

i)                    Mycobacterium tuberculosis is gram positive , both acid and alcoholic fast bacillus .

ii)                  Bovine type takes uniform stain .

iii)                Human type takes granular stain with thinner , longer and curved appearance .

iv)                After Ziehl – neelsen staining , when it is examined under microscope , the bacillus appears like red rod and the tissue are blue .

v)                  After staining with Auremin , a special stain , fluorescence appearance is shown by the organism in ultraviolet red . 

Spreading of Tubercle bacilli

   These disease may spread through three routes

i)                    Directly along continuous surface

ii)                  By the lymph system

iii)                By blood stream

1.. Spreading from continuous surface

(a)    When the lesions in the lung or bronchial glands are broken , there may be tuberculous broncho-pneumonia from where upper respiratory tract and face may be affected . Apex of right lung is commonly affected due to restricted movement of apical part and slowing of blood and lymphatic flow .

(b)    From the tuberculosis of epididymis , prostate and bladder may be affected due to ascending infection

2.. By the lymph stream

   The regional lymph glands draining the area are inflamed and caseous . It is found commonly in children . there may be lymphangitis of the lymphatics draining that area . There will be formation of primary complex which will consist of –

a)      the area of broncho-pneumonic patch in the lung

b)      Lymphangitis

c)      Glandular component like gland involvement .

In majority of the cases , it is arrested , being encapsulated by fibrous tissue and may even calcified . A few cases show other glandular group in volvement and enter into thoracic duct and then enter into blood stream .

3… By the blood stream

(a)    large number of organism when they enter into blood stream , cause generalized infection with production of military tuberculosis . Here small tubercle are present in many organs .

small number of organism when they enter into blood stream , localize themselves in various organs particularly the bones , joints and kidneys . this will produce chronic or subacute type of lesion .

Sources of Tuberculosis

i)                    Foelus -  Transplacental variety .

ii)                  Newborn baby – From and enviorment .

iii)                Infants –

(a)    From parents

(b)    From cow’s milk .

(c)    From bad environment .

iv)                Adults-

(a)    Bad effect of civilization .

(b)    Bad habits like spitting in public .

(c)    From coughing , sneezing and spitting by an infected person .

(d)   Handling of a T.B patient .

(e)    From the utensils of an infected person such as soiled cups , sauces , spoons etc .

Portals of entry of Mycobacterium Tuberculosis

The pathogenicity occurs in the human b eing from the following portal of entry of infection

   1.. The respiratory tract –

A health man can be infected by droplet infection by sneezing and coughing by the patient with open type of tubercular lesion . This will  cause Pulmonary Tuberculosis . right lung is mainly affected because right bronchus is short and straight and apex of the right lung is commonly affected due to restricted movement of the apical part and slowing of blood and lymphatic flow .Different opinions have expressed for lung infection from tubercle bacilli .

  2.. The alimentary tract –

   Through mouth and nasopharynx , tubercle bacilli may enter into alimentary tract and after absorption through the intestinal mucosa , tubercle bacilli entr into the lymph channel and blood and cause enlargement of cervical gland s and Peyer’s patches of small intestine from where they can pass to mesenteric glands . From infected milk , they may come into intestine .

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3… The skin .

   Through minute abrasion  in the skin , tubercle bacilli  may enter into body . Primary tuberculosis  of skin usually takes following forms –

i)                    Lupus vulgaris

ii)                  Verruca tuberculosa . 

Pathogenesis of tubercle bacillus

1. From pulmonary tuberculosis infection may go to mediastinal group of lymph nodes and cause tuberculosis of mediastinal lymph nodes . The enlarged lymph node will compress  a lung bron chous and produce atelectasis   or there may be ulceration in bronchial wall . Discharged caseous material from the ulceration into the lumen will produce extensive lesion  .

2.  From the tuberculosis of mediastinal glands pleura may be affected through lymphatics and there wil be formation of tuberculosis pleurisy which may be accompanied by effusion , called tuberculous empyema . pericardium may be affected and produce tuberculous pericarditis .

3. From primary lesion in tonsil , cold abscess of neck may be produced .

4. From gut , tuberculous peritonitis may be produced .

5. If tuberculosis lymph node ruptures , the bacilli may enter into the vein and they will produce acute miliary tuberculosis . Tubercuar meningitis often accompanies this condition .

     6.    Through blood stream , bacilli may be localized in various organs particularly bones ,, joints               and kidneys . This will produce chronic or subacute lesion .

Morphology of Mycobacterium tuberculosis

i)                    Usually straight or slightly curved slender rods with rounded ends . Clubbed and swollen forms are seen in culture .

ii)                  Arranged singly or in groups . Branching is rare .

iii)                Average size is 0.3 to 0.5μ in diameter and 1.2 to 4 μin length .

iv)                It’s appearance is granular or beaded .

v)                  Gram positive .

vi)                Non-sporing .

vii)              Non-capsuated .

viii)            Acid-fast .

ix)                Non motile due to absence of flagellum .