Corynebacterium diphtheriae

Corynebacterium is the causative agent of Diphtheria. It was discovered by bacteriologists Edwin Klebs and Frederich Loffler hence it is also called Klebs-Loffler’s bacillus.

Morphology

They are Gram positive , non sporing, non capsulated bacilli. They are arranged characteristically in Chinese letter patterns like letter V, L or in palisades. They appear club shaped due to the presence of granules composed of polymetaphosphate which stain purplish with methylene blue stain. These are seen only in pathogenic C.diphtheriae. The granules are energy storehouses for the bacteria.

Classification

Mc Leod classified Corynebacterium diphtheriae on the basis of colony characters as Gravis, Intermedius and Mitis with Gravis being most virulent and Mitis least virulent of the three.

Human Pathology

Pathogenicity is due to exotoxin. The toxin inhibits protein synthesis by inhibiting elongation factor 2 or EF 2. The toxin is coded by the tox gene which is carried by a lysogenic phage. If phage is lost so is toxicity. The toxin causes tissue necrosis causing pseudomembrane formation and it enters the bloodstream causing a toxemia primarily affecting the heart, nerves and adrenals.

Pseudomembrane is typically seen over the tonsils and throat, which may cause mechanical obstruction to breathing and asphyxia. Toxin may cause myocarditis and neuropathy with weakness and paralysis.

A rare form of diphtheria is cutaneous diphtheria which presents with cutaneous ulcers and pseudomembranes.

Laboratory diagnosis of Diphtheria

Swabs can be collected from tonsillar and pharyngeal areas with pseudomembranes. Smears are stained with Gram stain, methylene blue or Albert’s stain (for granules) and immunofluorescence. Culture is done on blood agar, Loeffler’s serum slope and tellurite agar (selective medium for C. diphtheriae). It shows black colonies on tellurite agar.

Demonstration of toxin production is essential for the microbiological diagnosis of C.diphtheriae as similar looking commensals are present in the human throat. Animal inoculation, Elek’s gel precipitation tests (positive test shows an arrow shaped precipitate), tissue culture or PCR for tox gene can be used for toxin detection.