CORYNEBACTERIUM DIPHTHERIAE

                      Corynebacterim diphtheriae is a club shaped involution form or slender rod shaped(normal form), non motile, non capsulated, non sporing and gram positive bacteria. they are pleomorphic and measuring approximately 3-6muv  m into 0.6-0.8muv.The bacilli may exist in pairs or groups.They commonly show chinese letter pattern(V, L, X)or cuniform arrangement.The cells shoe septa and 'polymeta phosphate granules' in the cytoplasm when stained with Loeffler;s methylene blue, the granules take up a bluish purple colour and hence they are called meta chromatic granules. They are also called volution or Babes Ernst granules.Diphtheria bacilli is classified into 3 types on the clinical severity.
                                                       1)Gravis

                                                       2)Intermedius

                                                       3)Mitis.

Diphtheria is a disease caused by the corynebacterium diphtheriae found in children.It is characterised by sore throat, fever, fatigue, malaise, pseudo membrane formation of tonsils and throat adenitis etc.

The disease was first recognised as a clinical entity by Bretonneau who called it 'diphtherite'.The name is derived from the tough, leathery, pseudo membrane formed in the disease.(Diphtheros means leather).Diphtheria bacillus was first observed and describad by klebs.It was cultivated by Loeffler bacillus.Roux and Yersin discovered diphtheria exotoxin and established its pathogenic effect.

CULTURAL CHARACTERISTICS:

                   Growth is scantu on the ordinart madia .Enrichment with blood, serum or egg is necessary for good growth.The optimum temperature for growth is 37 degree centigrade and optimum pH is 7.2. It is an aerobe and facultative anaerobe.On loeffler's serum agar medium, the bacteria out grows as small granular, moist, creamy colomies which gives a typical morphology. On potassium tellurite blood agar medium, they selectively grow and form black colour colonies.

BIO-CHEMICAL REACTIONS:

                  Diphtheria bacilli ferment glucose,galactose,maltose and dextrin but not lactose , mannitol, or sucrose. On fermentation they produce acid but no gas.These are catalase positive.They reduce nitrates and donot hydrolyse gelatin.

RESISTANCE:

            Diphtheria bacilli are destroyed by heat at 58 degree centigrade in 10 min and at 100 degerr centigrade in 1 min and are easily destroyed by antiseptics. They are susceptible to pencillin,erythromycin and broad spectrum antibiotics.

TOXIN:

             Virulent strains of diphtheria bacilli produce a very powerful exotoxin and it is more antigenic. Production of diphtheria exotoxin is a result of tox* gene.The gene is carried by the bacteriophage is known as beta-phage.C.diphtheriae strains are lysogenic for this phage.
           The diphtheria toxin is a protein and it has a molecular weight of about 62,000.It is extremely potent and the lethal dose for a 250g guinea pig is 0.0001mg. The toxin is labile and can be converted to toxoid at 37 degree centigrade for 4-6 weeks and treating it with 0.2-0.4% formalin.The diptheria toxin act by inhibiting protein synthesis in cardiac muscle, resulting in both structural and functional damage. Cardiac insufficiency can cause death.Demyelination caused by the toxin can affect both peripheral, cranial nerves and result in paralysis.

PATHOGENICITY;
                     The incubation period in diphtheria is commonly 3-4 days, but may on occasion be as short as one day.C.diphtheriae enters the body through respiratory track by inhalation.
          The site of infection may be faucial, laryngeal, nasal, otitis, conjuctival, genital and cutaneous.After entry they establish in the upper respiratory track and multiply usually in the nose or throat.They have very little invasive ability and rarely enters into the blood or tissues and they release powerful exotoxin.Exotoxin consists of 2 fragments A and B. The 'B' fragment attaches to specific receptors on the cell membrane of the host cell and toxin is taken into the cell by endocytosis.
                    After the entry into the cell, the toxin is activated by protease,the 'A' fragment seperate from the 'B' fragment and becomes an active enzyme.This enzyme tranfers a protein of NAD of the cell on to an elongation factor -2(EF-2).which is essentially irreversible.It activates the EF-2 and there by stops protein synthesis.This causes local necrosis and th eresulting fibrinous exudate together with disintegrating epithelial cells, leucocytes and erythrocytes and bacteria formed a grey  white pseudomembrane in the throat which is characteristic feature of diphtheria infection.

CLINICAL MANIFESTATIONS:

         Sore throat, fever, fatigue, malaise, pseudomembrane formation on tonsils and throat, marked adenitis(bullneck), cellulitis, asphyxia, acute circulatory failure, otitis media, pneumonia,heart and kidney failure etc...

LABORATORY DIAGNOSIS:

Laboratory diagnosis consists of  isolation od diphtheria bacillus and demonstration of its toxicity.

              A)ISOLATION OF DIPHTHERIA BACILLUS:
                            a)MICROSCOPY: Specimens are collected from the throat swabs using tongue depressor and smear is preapred.After gram staining, it is observed under microscope for diphtheroids.The smear may also be stained with loeffler's methylene blue stain and observed under microscope.Its typical morphology identifies corynebacteria.
                            b)CULTUTE: For cluture, the swabs are inoculated on loeffers serum slope,potassium tellurite blood agar.On Loeffler's serum slope ,growth is observed within 6-8hrs and on potassium tellurite blood agar, grey or black colonies are observed.

              B)DEMONSTRATION OF TOXICITY:
                              a)ELEK'S GEL PRECIPITATION TEST:
                                           A rectangular strip of filter paper impreganated with diphtheria antitoxin (100units/ml)is placed on the surface of 20% normal horse serum agar in petridish.Testing strain is inoculated at right angle to filter paper strip and incubated at 37 degree centigrade  for 24 -48 hrs.Toxin produced by bacterial growth will diffuse into agar and produce line of precipitation .They are compared with controls.No precipitate will occur in non toxigenic strain.

EPIDEMIOLOGY:

          Diphtheria has been virtually erdicaated from most advanced countries.Thus in england and wales , between 1915 and 1942 , the number of diphtheri cases per year was about50,000 and deaths around 2,500-4,000.It  was the commonest cause of death in children aged 4-10 yrs.It is rare in the first year of life, reaches a peak between 2 - 5 years, falls  slowly between 5-10 years an drapidily between 10- 15 years.Infection is rare in early infany because of the passive immunity obtained from the mother.The disease is commoner in rural than in urban years. Carriers transmit the infection  to their contacts.Fomites do not seem to play an important role though in special situations toys and pencils may act as vehicles of infection.

PROPHYLAXIS:

                        General control methods comprise early diagnosis, prompt hospitalization, recognition of carriers.Diphtheria can by a programme of mass imunisation , for this Diphtheria-Pertusis-Tetnus(DPT) vaccine is used.Booster immunisation at 10 year intervals maintain immunity.

                The methods of immunisation available are active, passive or combined.Susceptibility to diphtheria can be detected by the schick test when the diphtheria toxin is injected intradermally into a susceptibility person , it causes a local reaction while in an immune individual, no reaction ensures as the toxin is neutralised by the antitoxin in blood circulation.

 TREATMENT:
       Specific treatment of diphtheria consists of antitoxic and antibiotic therapy.
 C.diphtheria is sensitive to pencillin and can be cleared from the throat within few days by pencillin treatment.