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

Morphology

They are Gram-positive, non-sporing, non-capsulated bacilli. They are characteristically arranged in Chinese-letter patterns (such as V or L shapes) or in palisades.

They appear club-shaped due to the presence of granules composed of polymetaphosphate, which stain purplish with methylene blue. These granules are seen only in pathogenic C. diphtheriae. The granules act as energy storehouses for the bacteria.

Classification

Mc Leod classified Corynebacterium diphtheriae on the basis of colony characteristics into Gravis, Intermedius, and Mitis. Of these, Gravis is the most virulent and Mitis is the least virulent.

Human pathology

Pathogenicity is due to an exotoxin. The toxin inhibits protein synthesis by inhibiting elongation factor 2 (EF 2). The toxin is coded by the tox gene, which is carried by a lysogenic phage. If the phage is lost, toxicity is also lost.

The toxin causes tissue necrosis, leading to pseudomembrane formation. It can also enter the bloodstream and cause toxemia, primarily affecting the heart, nerves, and adrenals.

Pseudomembrane is typically seen over the tonsils and throat. This may cause mechanical obstruction to breathing and lead to asphyxia. The toxin may also 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 (a 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, because similar-looking commensals are present in the human throat. Animal inoculation, Elek’s gel precipitation test (a positive test shows an arrow-shaped precipitate), tissue culture, or PCR for the tox gene can be used for toxin detection.