Biotypes of Corynebacterium Diphtheriae

Corynebacterium diphtheriae is a nonmotile, noncapsulated, club-shaped, gram-positive bacillus.

Toxigenic strains are lysogenic for one of a family of corynebacteriophages that carry the structural gene for diphtheria toxin, tox.

Corynebacterium diphtheriae is classified into biotypes (mitis, intermedius, and gravis) according to colony morphology, as well as into lysotypes based upon corynebacteriophage sensitivity.

Biotypes of Corynebacterium diphtheriae

McLeod and Anderson classified diphtheria bacilli, based on the colony characteristics on Tellurite medium and other properties like biochemical reactions and severity of disease.

Properties

Gravis

Intermedius

Mitis

 

Morphology

– Short Rods

– Uniform Staining

– Few or no granules

– May be pleomorphic

– Long-barred forms, clubbed end

– Poor Granulation

– Very pleomorphic

– Long Rods

– curved shaped

– pleomorphic

– Prominent granules

Colony on Tellurite Blood Agar ≥ 2mm, dull greyish black, opaque colonies, daisy head, brittle, like cold margarine. < 0.5 mm, gray colony, dark center shining surface, frog’s egg colonies ≥ 2mm, gray, opaque glossy, smooth surface, poached egg colonies, soft buttery, easily emulsifiable
Hemolysis Variable Non-hemolytic Usually Hemolytic
Glycogen and Starch fermentation Positive Negative Negative
Antigen Types 13 4 40
Fatality Rate High High Low
Complications Paralytic Hemorrhagic Obstructive
Endemicity Epidemic Epidemic Endemic

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