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Genus Corynebacterium

Gram positive rods
Spore forming
Non spore forming
Anaerobic
Aerobic
Clostridium spp
Bacillus spp
Corynebacterium
Gram Positive Bacilli

Species of Corynebacteria

Normal flora of RT, urethra, vagina, Skin
Other Significant Corynebacterium species C. xerosis C. pseudodiphtheriticum C. pseudotuberculosis C. jekeium, (skin) C. ulcerans
Corynebacterium diphtheriae



Gram positive, pleomorphic non motile bacilli with irregularly stained segmentsFrequently show club shaped swellings – corynebacteria (coryne = club)stains more intensely at its ends, which contain volutin granules (Babes-Ernst granules, metachromatin)C. diphtheriae : most important member of this genus, causes diphtheriaDiphtheroids : commensals of nose, throat, nasopharynx, skin, urinary tract & conjunctiva. Causes infections in immunocompromised. Characters

C.diphtheriae

Exotoxin – Diphtheria toxin:Protein in naturevery powerful toxinResponsible for all pathogenic effects of the bacilliProduced by all the virulent strains

Pathogenesis

Virulent bacilli lodge in throat of susceptible individual Multiply in superficial layers of m.m Elaborate toxin which causes necrosis of neighboring tissue cells Inflammation eventually results in pseudomembrane (fibrinous exudate with disintegrating epithelial cells, leucocytes, erythrocytes & bacteria) Appears first on tonsils or posterior pharynx and spreads upward or down Regional lymphnodes in neck often enlarged (bull neck)

Non-Respiratory Disease

Systemic infections Toxin is absorbed in the blood stream and carried systemically Affects the kidneys, heart, and nervous system Death occurs due to cardiac failure

Laboratory Diagnosis

Specimen – swab from the lesionsMicroscopy Gram stain: Gram +ve bacilli, chinese letter patternAlbert’s stain for metachromatic granules


Albert’s Staining Alber’s stain 3-5 min.Wash with water.Albert’s iodine 1 min.Wash with water.Results :light green bacilli with bluish black granules.

Laboratory Diagnosis

Culture – isolation of bacilli requires media enriched with blood, serum or eggEnriched media:Blood agar, loeffler’s medium Selective media: potassium tellurite agar – tellurite is reduced to telluride (black), gives gray or black color to the colonies


The colonies of C. diphtheriae are small, granular, grey, smooth, and creamy with irregular edges

Growth of diphtheria bacilli

Laboratory Diagnosis
3. Virulence tests - Test for toxigenicityInvivo tests – animal inoculation (guinea pigs)Invitro testsElek’s gel precipitation test

Laboratory Diagnosis Virulence tests - Invivo tests

Bacterial growth from is emulsified in 2-4 ml broth. Intradermal inoculation of an animal (guinea pig) The appearance of inflammation, oedema, and necrosis at the site of inoculation.

Detection of toxin in vitro: Elek’s Test Principle: It is toxin/antitoxin reaction Toxin production by C.diphtheriae can be demonstrated by a precipitation between exotoxin and diphtheria antitoxin Procedure: A strip of filter paper impregnated with diphtheria antitoxin is placed on the surface of serum agar The organism is streaked at right angels to the filter paper Incubate the plate at 37C for 24 hrs


Resuls: After 48 hrs incubation, the antitoxin diffusing from filter paper strip and the toxigenic strains produce exotoxin, which diffuses and resulted in lines four precipitation lines radiating from intersection of the strip and the growth of organism
Filter paper saturated with diphtheria antitoxin
Inoculated M.O.
Positive Elek’s Test Lines of precipitations

Toxigenic corynebacterium


Non-toxigenic corynebacterium


4) PCR: For detection of the diphtheria toxin genes. 5) ELISA: For detection of the diphtheria toxin. 6) Tissue culture test: The test strain is inoculated into an agar overlay of cell culture. The toxin produced diffuses into the cells below and kill them.

Other Corynebacteria

Diphtheroids – Normal commensals of nose, throat, nasopharynx, skin, urinary tract & conjunctivaStain uniformlyFew or no metachromatic granulesArranged in parallel rows (palisades)Nontoxigenic

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