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 .

Diagnosis of Mycobacterum tuberculosis

         I  Direct method

A ..  Microscopical examination

      (a  A portion of mucopurulent material will be taken on a grease-free glass slide and with the help of platinium wire with loop , the smear is prepared and it is stained b e ziehl-neelsen technique .

(b)    Antiformin method ( concentration technique ) 

               Composition of Antiformin –

i)                    Liquid sodium chlorinate – 1 part

ii)                  15% sodium hydroxide -  1 part

iii)                Distilled water – 8 part .

 Advantage of Antiformin –

i)                    It dissolves the mucous

ii)                  It kills the other group of organisms .

   Techniques …..

i)                    One part of sputum is added with three part of Antiformin  (20%) and mixed thoroughly .

ii)                  The mixture is inoculated at 37*c for 30 minutes .

iii)                Then it is centrifuged for 30 minutes . The deposit is used for smear and staining iis done by Ziehl-Neelsen technique .

iv)                Petroff’s method ( concentration technique )  …..

     Technique -----

i)                    one volume of sputum is added with 4 volume of 4% Caustic soda and mixed thoroughly .

ii)                  The mixture is kept in incubator at 37*c for 30 minutes and ashaken from time to time .

iii)                Then the mixture is centrifuged at 3000 r.p.m

iv)                The supernatant fluid will be poured off

v)                  The deposit will be treated with normal saline and neutralized with 8% hydrochloric acid or dilute aceticacid solution . Then it will be centrifuged .

vi)                The deposit is used for smear and stained by ziehl-neelsen technique .

Advantages of Petroff’;s method

i)                    Caustic soda will kill the other organism and liquefy the mucous .

ii)                  The deposit may be used for staining by ziehl neelsen technique , for culture in Lowenstein Jensen’s medium and for inoculation in guinea pig .

B…    Culture -  Lowenstein jensen’s medium and Dorset’s egg medium are used for the culture .

C..  Animal pathogenicity test – rabbit and guineapig are the animals of choice . here concentrated                                                           material ( Antiformin method or petroff method ) is inoculated to the                                                     animal of choice . The animal will die in 12 weeks ( in case of bivine                                                     type ) and 3 to 9 months ( in case of human type ) . Lungs and                                                               kidney will be affected in case of rabbit and spleen and liver will be                                                       affected in case of guinea-pig .

II  .. Indirect nmethod ----

i)                    Allergic reaction like mantoux test

ii)                  Complement fixation test

iii)                Increase in sedimentation of blood corpuscles .

iv)                Laevodeviation of Arneth index .

Chemical composition of Mycobacterium tuberculosis

i)                    Mykol – This is responsible for acid fast staining .

ii)                  Lipoid –

(a)    Glyceroid

(b)    Phospatids

(c)    Wax

iii)                Polysaccharides -  These are responsible for necrosis caseation and tubercle formation .

iv)                Protein – this is responsible for immunity and allergy .