Diphtheria agent

Morphology and staining properties

The causative agent is a thin, straight, or slightly curved pleomorphic rod. They are non-motile and non-spore-forming. The cell wall contains coryneform mycolic acids (lipids), which provide high resistance of the bacterium in the external environment. Many strains have a microcapsule.

A distinctive feature of C. diphtheriae is the presence of intracellular inclusions at the poles called volutin granules (Babes–Ernst granules). They give the bacterium a club-like shape. When using special stains (Neisser or Loeffler), these granules exhibit metachromasia, staining more intensely than the rest of the cytoplasm. Unlike normal microflora (coryneform bacteria), which arrange in a smear parallel to each other ("palisades"), the diphtheria pathogen is arranged haphazardly, forming shapes resembling "spread fingers".

Cultural and biochemical characteristics

The bacterium is a facultative anaerobe and has high nutritional requirements. Serum, blood, and amino acids are necessary for its growth. Laboratories use Roux's medium or tellurite media (e.g., Clauberg II).

The addition of potassium tellurite inhibits the growth of 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. Forms a pellicle on broth and ferments starch.
• Biovar mitis (mild): forms small, smooth (S-form), shiny colonies. Produces uniform turbidity in broth; does not ferment starch.

Biochemically, all strains ferment glucose and maltose, but do not affect sucrose. The most important diagnostic sign of the species is cystinase (+) (blackening on Pisu medium) and the absence of urease activity.

Pathogenicity factors and exotoxin

The aggressiveness of the bacterium is provided by microcapsule components (cord factor, K antigens) and enzymes (hyaluronidase, neuraminidase, dermonecrotoxin). However, the main pathogenicity factor is diphtheria histotoxin. The ability to produce it is present only in strains infected with a specific prophage (carrying the tox gene).

The toxin consists of two fragments:

1. B-fragment — binds to target cell receptors (myocardium, nerves) and forms a channel.
2. A-fragment — penetrates the cytoplasm, where it blocks elongation factor EF-2. This irreversibly halts protein synthesis on ribosomes, leading to target cell death.

Pathogenesis and clinical presentation

The disease develops as a toxaemic infection. The portals of entry are usually the mucous membranes of the pharynx and nose.

The process of specific diphtheria pseudomembrane formation:

• Dermonecrotoxin kills epithelial cells, releasing tissue thrombokinase.
• Hyaluronidase increases vascular permeability, and fibrinogen enters the tissue.
• Upon contact between fibrinogen and thrombokinase, a network of fibrin is formed.

Clinically, this manifests as pain upon swallowing, lymphadenopathy, and the appearance of dense grayish pseudomembranes on the tonsils. An important sign: the membrane is tightly fused to the tissue, cannot be removed with a cotton swab, and attempts to tear it off cause the mucosa to bleed.

Principles of diagnostics, treatment, and prevention

The main diagnostic method is bacteriological. The swab is taken with a dry swab strictly before starting antibiotics and transported to the laboratory using glycerol. To detect the toxin, indirect hemagglutination assay (IHA), ELISA, and genetic diagnostics (PCR for the tox gene) are used.

Treatment is based on the early administration of equine diphtheria antitoxin (or immunoglobulin) to neutralize the toxin. A skin test is mandatory prior to administration to prevent anaphylaxis. Antibiotics (penicillins, erythromycin) are additionally prescribed.

Prevention is carried out through routine vaccination with preparations based on diphtheria toxoid (DTP, DT, Td). It induces long-lasting antitoxic immunity.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed differences between gravis, mitis, and intermedius biovars on media
• Laboratory models for determining strain toxigenicity (including in vitro methods)
• Molecular mechanism of EF-2 elongation factor blockade by the exotoxin
• Differential diagnosis with normal coryneform flora (diphtheroids)
• Rules for administering diphtheria antitoxin according to Besredka

Other topics in the section "Irregular Gram-positive rods (Diphtheria, Mycobacteria/tuberculosis and leprosy, Actinomycetes, Nocardia, Bifidobacteria, Gardnerella, etc.)"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.