Morphology and Staining Properties
The pathogen appears as thin, straight or slightly curved polymorphic rods. They are non-motile and do not form spores. The cell wall contains coryneform mycolic acids (lipids), providing high resistance to environmental factors. Many strains possess a microcapsule.
A distinctive feature of C. diphtheriae is the presence of polar bodies known as volutin granules (Babès-Ernst granules). These give the bacteria a club-like shape. When using special stains (such as Neisser or Loeffler stains), these granules demonstrate metachromasia, staining more intensely than the rest of the cytoplasm. Unlike normal microflora (coryneform bacteria), which line up in smears parallel to each other ("palisades"), the diphtheria pathogen is arranged chaotically, forming figures resembling "spread fingers".
Cultural and Biochemical Characteristics
The bacterium is a facultative anaerobe and has high nutritional requirements. Serum, blood, and amino acids are required for its growth. Laboratories use Roux medium or tellurite-containing media (such as Clauberg II).
The addition of potassium tellurite inhibits accompanying flora. The diphtheria pathogen reduces it to metallic tellurium, causing colonies to turn black. Several biovars are distinguished, the main ones being:
- Biovar gravis (severe): forms large, rough (R-form) colonies resembling a daisy flower. It produces a pellicle in broth and ferments starch.
- Biovar mitis (mild): forms small, smooth (S-form), shiny colonies. It causes uniform turbidity in broth and does not ferment starch.
Biochemically, all strains ferment glucose and maltose, but not sucrose. The most critical diagnostic criterion for the species is cystinase (+)* (blackening on Pisu medium) and the absence of urease activity.
Pathogenicity Factors and Exotoxin
Bacterial aggressiveness is mediated by microcapsular components (cord factor, K-antigens) and enzymes (hyaluronidase, neuraminidase, dermo-necrotoxin). However, the primary pathogenicity factor is the diphtheria histotoxin. The ability to produce it is restricted to strains infected with a specific prophage carrying the tox gene.
The toxin consists of two fragments:
- B-fragment — binds to receptors on target cells (myocardium, nerves) and forms a channel.
- A-fragment — enters the cytoplasm, where it blocks elongation factor 2 (EF-2). This irreversibly halts protein synthesis on ribosomes, leading to target cell death.
Pathogenesis and Clinical Presentation
The disease develops as a toxin-mediated infection. The typical port of entry is the mucous membranes of the pharynx and nose.
Mechanism of specific diphtheritic membrane formation:
- Dermo-necrotoxin kills epithelial cells, releasing tissue thromboplastin.
- Hyaluronidase increases vascular permeability, causing fibrinogen to leak into the tissues.
- Contact between fibrinogen and thromboplastin generates a network of fibrin.
Clinically, this manifests as odynophagia (painful swallowing), lymphadenopathy, and the appearance of dense grayish pseudomembranes on the tonsils. A key diagnostic sign: the membrane is tightly adherent to the underlying tissue, cannot be removed with a cotton swab, and attempts to strip it cause the mucosa to bleed.
Principles of Diagnosis, Treatment, and Prevention
The primary diagnostic method is bacteriological culture. Swabs must be taken with a dry swab strictly before antibiotic therapy begins and transported to the laboratory using glycerol. To detect the toxin, passive hemagglutination inhibition assays (PHA), ELISA, and genetic diagnostics (PCR for the tox gene) are used.
Treatment is based on the prompt administration of equine antidiphtheria serum (or immunoglobulin) to neutralize the toxin. A skin test must be performed prior to administration to prevent anaphylaxis. Antibiotics (penicillins, erythromycin) are prescribed adjunctively.
Prevention is carried out via routine vaccination using preparations based on diphtheria toxoid (e.g., DTaP, DT). This induces robust antitoxic immunity.