Case 75. Diphtheria (68 y-o M)
Pseudomembrane formed in the throat in a patient with fatal dyspnea
Key words : Corynebacterium diphtheriae, pseudomembrane
The pharyngeal squamous mucosa is densely covered with faintly basophilic material (pseudomembrane). Moderate submucosal inflammation is seen here (HE).   Another view of the pharyngeal mucosa demonstrates thick pseudomembrane attachment and relatively mild inflammatory reaction (HE).
Numbers of large-sized Gram-positive rods are embedded within the pseudomembrane (Gram).   Scanning electron micrograph of a club-shaped rod without flagella or capsule, being consistent with the Corynebacterium species ("coryne" means club).
Cultured Corynebacterium diphtheriae (Neisser stain)
Metachromatic granules, stained black-brown, are seen at one end or both ends of the rods. The pathogenicity of C. diphtheriae is determined by diphtheria toxin production.
  Non-pathogenic Corynebacterium in conjunctival mucosa (Quick Giemsa)
Club-shaped, large-sized rods are often seen in a Y or V-shaped (non-parallel) arrangement. Non-diphtherial Corynebacteria are non-pathogenic, and are distributed in the oral cavity, skin, conjunctiva and vagina as normal flora.