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26
Feb
Corynebacterium diphtheriae biovar intermedius
Microbiology, Pathogenesis, and Clinical Significance
Quick Summary
Corynebacterium diphtheriae biovar intermedius represents a crucial phenotypic variant of the diphtheria pathogen. Known for its distinct colony morphology and intermediate virulence, this biovar plays a significant role in clinical diagnostics and global epidemiological surveillance.
1 Taxonomic Position & History
Corynebacterium diphtheriae is the most clinically significant species within its genus. Historically, based on colony morphology and biochemical behavior, it was classified into three major biovars:
- Gravis: Associated with severe outbreaks.
- Mitis: Typically leads to milder localized infections.
- Intermedius: Represents the intermediate phenotype between the two.
2 Biological Characteristics
Under microscopy, biovar intermedius appears as Gram-positive, pleomorphic, club-shaped rods. Key features include:
- Metachromatic Granules: Also known as Babes–Ernst granules, revealed by Neisser staining.
- Cell Arrangement: Characteristic angular V-shaped or palisade patterns.
- Culture: Small (0.5–1 mm), flat, gray-to-black colonies on tellurite media.
Fig 1. Microscopic morphology and colony characteristics.
Pathogenic Mechanisms
The virulence is primarily driven by the Diphtheria Toxin (DT), an AB-type exotoxin. Notably, this is a result of lysogenic conversion by a bacteriophage carrying the tox gene.
Mechanism of Action:
The A subunit catalyzes the ADP-ribosylation of Elongation Factor-2 (EF-2), irreversibly halting protein synthesis and leading to cellular death in myocardial and neural tissues.
Clinical Manifestations & Diagnosis
Diphtheria caused by biovar intermedius is characterized by the formation of a gray-white pseudomembrane. If left untreated, it can lead to airway obstruction or toxic myocarditis.
| Diagnostic Stage | Method/Key Observations |
|---|---|
| Initial Screening | Direct microscopic examination (Club-shaped bacilli). |
| Gold Standard | Culture on Löffler’s serum or tellurite blood agar. |
| Toxigenicity | Elek test (Immunodiffusion) or PCR for tox gene. |
Treatment and Prevention
Medical Management
Immediate Antitoxin administration is critical. Supplemental antibiotics (Penicillin/Erythromycin) are used to eliminate the bacteria.
Public Health
The Diphtheria Toxoid vaccine (DTaP/Tdap) remains the most effective preventive measure to maintain herd immunity.
Biofargo team
