Corynebacterium diphtheriae

Corynebacterium diphtheriae
Scientific classification
Domain:	Bacteria
Phylum:	Actinobacteria
Order:	Actinomycetales
Family:	Corynebacteriaceae
Genus:	Corynebacterium
Species:	C. diphtheriae
Binomial name
Corynebacterium diphtheriae

Corynebacterium diphtheriae is the pathogenic bacterium that causes diphtheria. It is also known as the Klebs-Löffler bacillus, because it was discovered in 1884 by German bacteriologists Edwin Klebs (1834–1912) and Friedrich Löffler (1852–1915).

Four subspecies are recognized: C. d. mitis, C. d. intermedius, C. d. gravis, and C. d. belfanti. The four subspecies differ slightly in their colonial morphology and biochemical properties, such as the ability to metabolize certain nutrients, but all may be toxigenic (and therefore cause diphtheria) or not toxigenic. C. diphtheriae produces diphtheria toxin which alters protein function in the host by inactivating the elongation factor EF-2. This causes pharyngitis and 'pseudomembrane' in the throat. The diphtheria toxin gene is encoded by a bacteriophage found in toxigenic strains, integrated into the bacterial chromosome.

To accurately identify C. diphtheriae, a Gram stain is performed to show Gram-positive, highly pleomorphic organisms with no particular arrangement. Special stains like Alberts's stain and Ponder's stain are used to demonstrate the metachromatic granules formed in the polar regions. The granules are called as polar granules, Babes Ernst granules, volutin, etc. An enrichment medium, such as Löffler's medium, is used to preferentially grow C. diphtheriae. After that, a differential plate known as tellurite agar, allows all Corynebacteria (including C. diphtheriae) to reduce tellurite to metallic tellurium. The tellurite reduction is colorimetrically indicated by brown colonies for most Cornyebacteria species or by a black halo around the C. diphtheriae colonies.

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