Corynebacterium diphtheriae: PCR Detection and Clinical Overview

C. diphtheriae is a Gram-positive, non-spore-forming rod with characteristic metachromatic granules visible by methylene blue staining.

Based on colony morphology, it is classified into three biotypes: Gravis, Intermedius, and Mitis, which differ in epidemiology but not significantly in toxin production.

The bacterium is resistant to desiccation and can survive on fomites such as clothing or toys for several weeks.

It is heat-sensitive and can be inactivated at 60°C within 10 minutes.

Selective media such as potassium tellurite blood agar are commonly used, where colonies appear black or gray-black.

Enrichment media such as Loeffler serum medium promote rapid growth and enhance visibility of metachromatic granules.

On blood agar, colonies are typically 1–2 mm in diameter, gray-white, and non-hemolytic or weakly hemolytic.

Transmission occurs primarily via respiratory droplets or direct contact with infected secretions.

Pharyngeal and laryngeal diphtheria account for approximately 95% of cases, characterized by gray pseudomembrane formation.

Forced removal of the pseudomembrane can lead to bleeding and airway obstruction.

“Bull neck” appearance may develop due to cervical lymphadenitis.

Systemic toxin dissemination can result in myocarditis (major cause of mortality) and peripheral neuropathy.

Cutaneous diphtheria presents as ulcerative lesions and is more common in tropical regions.

Microscopy: Albert staining reveals metachromatic granules as dark inclusions within blue-green rods.

Culture: Initial growth on Loeffler medium (24 hours), followed by subculture on tellurite agar (48 hours).

Toxin detection: Elek test identifies toxin production via precipitation lines in agar diffusion assays.

Molecular detection: PCR targeting the tox gene provides rapid and specific identification.

Effective physical methods include boiling at 100°C for 1 minute and autoclaving at 121°C for 15 minutes.

Chemical disinfectants such as chlorine-based agents (1000 ppm), 70% ethanol, and iodophors are also effective.

Contaminated materials should be thoroughly disinfected or sterilized to prevent transmission.

Diphtheria antitoxin (DAT) is essential to neutralize circulating toxin and should be administered promptly after diagnosis.

Antibiotics such as penicillin G or erythromycin are used to eliminate bacterial infection and prevent transmission.

Supportive care includes airway management and cardiac monitoring due to risk of respiratory obstruction and myocarditis.

The bacterium remains largely susceptible to these antibiotics, with minimal resistance reported.

Vaccination with diphtheria toxoid (DTaP) is the most effective preventive measure, requiring both primary and booster immunization.

Herd immunity coverage above 85% is necessary to prevent outbreaks.

Confirmed cases require isolation until two consecutive negative cultures (24 hours apart), and close contacts should receive prophylactic antibiotics.