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.
By teamBiofargo
BIOFARGO TEAM
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