Corynebacterium diphtheriae

Corynebacterium diphtheriae

1. Introduction to Corynebacterium diphtheriae

Corynebacterium diphtheriae is a Gram-positive, non-spore-forming, facultatively anaerobic bacterium that causes diphtheria, a potentially life-threatening infection. It primarily affects the upper respiratory tract, leading to the formation of a characteristic pseudomembrane in the throat and severe systemic toxicity. Although rare in industrialized nations due to widespread vaccination, diphtheria remains a significant cause of morbidity and mortality in areas with low vaccination coverage.

The pathogenicity of C. diphtheriae is largely due to the production of diphtheria toxin, which can spread systemically and damage tissues by inhibiting protein synthesis in host cells. The toxin can cause complications such as myocarditis, polyneuropathy, and respiratory failure if not treated promptly (Merritt et al., 2021).

2. Taxonomy and Classification

• Domain: Bacteria
• Phylum: Actinobacteria
• Class: Actinobacteria
• Order: Corynebacteriales
• Family: Corynebacteriaceae
• Genus: Corynebacterium
• Species: Corynebacterium diphtheriae

C. diphtheriae is part of the Corynebacterium genus, which contains several species responsible for human infections, including C. jeikeium and C. ulcerans. It is closely related to other diphtheria-causing species such as Corynebacterium pseudotuberculosis and Corynebacterium bovis (Tremblay et al., 2020).

3. Morphological Characteristics

• Shape and Arrangement: C. diphtheriae is a club-shaped bacterium with variable morphology. It is typically rod-shaped with a characteristic "palisade" arrangement or Chinese letter appearance when viewed under the microscope (Rath, 2019).
• The bacteria often form angular clusters, making them distinguishable from other bacteria in Gram stains.
• Gram Staining: C. diphtheriae is Gram-positive, with a thick peptidoglycan layer that retains the violet dye in Gram staining. The bacteria appear as purple rods under a microscope, often with a slight bulge or swelling at the ends (Bakkaloglu & Khan, 2017).
• Cell Wall Structure: The cell wall of C. diphtheriae contains mesodiaminopimelic acid and a complex layer of arabionogalactan and mycolic acids, which contribute to its cell wall integrity and pathogenic potential (Rath, 2019).

4. Cultural Characteristics

Corynebacterium diphtheriae is a slow-growing, facultatively anaerobic bacterium with specific cultural characteristics. It requires special growth conditions and media for effective isolation and differentiation from other organisms.

• Growth Media:
• C. diphtheriae grows well on blood agar and Löffler’s serum medium, which are commonly used for isolation. Blood agar provides nutrients and supports the growth of most bacterial species, while Löffler’s medium helps in the production of the pseudomembrane typical of diphtheria.
• It can also be cultured on Cystine-Tellurite Agar (CTA), where it produces gray-black colonies due to the reduction of tellurite (Bakkaloglu & Khan, 2017).
• Colony Morphology:
• On blood agar, colonies of C. diphtheriae are typically small, round, grayish-white, with a fuzzy or granular appearance. They are generally non-hemolytic but can show alpha-hemolysis under some conditions.
• On Cystine-Tellurite Agar, the colonies appear black due to the reduction of tellurite in the medium. This feature helps in distinguishing C. diphtheriae from other corynebacteria (Rath, 2019).
• Growth Requirements:
• The optimal temperature for growth is 35-37°C, which corresponds to body temperature. C. diphtheriae grows best under aerobic conditions but can also grow in anaerobic environments, albeit more slowly (Merritt et al., 2021).
• Biochemical Characteristics:
• Catalase Test: C. diphtheriae is catalase-positive, meaning it produces the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. This test helps differentiate it from some other bacteria (Bakkaloglu & Khan, 2017).
• Urease Test: It is urease-negative, which can help in differentiating it from other corynebacteria (Merritt et al., 2021).
• Nitrate Reduction: C. diphtheriae is positive for nitrate reduction, converting nitrate to nitrite (Rath, 2019).
• Sugar Fermentation: It can ferment glucose and sucrose but is typically non-fermentative with other carbohydrates like maltose and lactose (Merritt et al., 2021).
• Toxin Production:
• C. diphtheriae is distinguished from non-pathogenic corynebacteria by its ability to produce diphtheria toxin, which is encoded by the tox gene carried by a lysogenic bacteriophage (Blumberg et al., 2019). Toxin production is tested using the Elek test, a precipitation test where the bacterium is cultured on a plate with antitoxin, and toxin production is confirmed by the formation of a visible line of precipitation.

5. Virulence Factors

• Diphtheria Toxin:
• The primary virulence factor of C. diphtheriae is its diphtheria toxin, which is a potent AB exotoxin. The A subunit is the active enzymatic component, and the B subunit is responsible for binding to host cells and facilitating toxin entry.
• Mechanism of Action: Once internalized by host cells, the A subunit of the toxin inactivates elongation factor 2 (EF-2) by ADP-ribosylation, which inhibits protein synthesis. This leads to cell death and tissue damage, contributing to the formation of the characteristic pseudomembrane and systemic toxicity (Merritt et al., 2021).
• Toxin Receptors: The toxin binds specifically to heparin-binding epidermal growth factor (HB-EGF) receptors on host cells, facilitating its uptake by receptor-mediated endocytosis (Rath, 2019).
• Systemic Effects: Beyond local effects in the throat, the toxin can enter the bloodstream and cause systemic complications, including myocarditis, polyneuropathy, and even death if untreated (Blumberg et al., 2019).
• Iron Acquisition Mechanisms:
• C. diphtheriae has developed mechanisms to scavenge iron from its host, which is essential for its growth. These mechanisms include the production of siderophores and transferrin-binding proteins, which aid in acquiring iron from the host's immune system (Blumberg et al., 2019).
• Adherence Factors:
• The bacterium has surface proteins that facilitate adherence to epithelial cells in the upper respiratory tract, which is crucial for colonization and the formation of the pseudomembrane (Merritt et al., 2021).

6. Pathogenesis of Diphtheria

• Colonization and Local Effects: The infection begins in the nasopharynx or tonsils, where C. diphtheriae colonizes and produces the diphtheria toxin. The local effects include the formation of a grayish-white pseudomembrane composed of fibrin, dead cells, and bacteria. This membrane can obstruct the airway and cause severe respiratory distress (Blumberg et al., 2019).
• Systemic Spread: The toxin can enter the bloodstream, leading to toxic myocarditis, neuropathy, and other systemic complications. In severe cases, respiratory failure and death may result if not treated promptly (Rath, 2019).
• Immune Response: The immune response to diphtheria includes the production of antibodies against the diphtheria toxin. However, in the absence of prior immunization, the immune response is insufficient to prevent severe disease, and the systemic effects of the toxin dominate (Merritt et al., 2021).

7. Diagnosis

• Clinical Diagnosis: Diphtheria is suspected in patients with a sore throat, fever, swollen lymph nodes, and the presence of a pseudomembrane in the throat. It is often associated with respiratory distress due to the airway obstruction (Blumberg et al., 2019).
• Laboratory Diagnosis:
• Microscopy: The bacterium can be detected in clinical samples, such as throat swabs, using Gram stain or methenamine silver stain to reveal the characteristic club-shaped rods (Rath, 2019).
• Culture: Isolation of C. diphtheriae from throat swabs is done on selective media like Löffler’s serum medium and Cystine-Tellurite agar, where the bacteria produce characteristic colony morphology (Merritt et al., 2021).
• Toxin Detection: Detection of diphtheria toxin can be done by Elek test, PCR, or toxigenic culture, all of which confirm the presence of the toxin (Bakkaloglu & Khan, 2017).

8. Treatment

• Antitoxin: The primary treatment for diphtheria is the administration of diphtheria antitoxin to neutralize the circulating toxin and prevent further systemic damage (Merritt et al., 2021).
• Antibiotics: Penicillin or erythromycin are used to eradicate the bacterial infection and reduce transmission. The antibiotic treatment also helps reduce the bacterial load, minimizing the production of further toxin (Rath, 2019).

9. Prevention

• Vaccination: The diphtheria vaccine, which is part of the DTP (diphtheria, tetanus, and pertussis) vaccine series, is highly effective in preventing diphtheria. The vaccine induces an immune response against the diphtheria toxin and protects against both local and systemic effects of the infection (Blumberg et al., 2019).
• Booster Doses: Booster doses of the diphtheria vaccine are recommended for adults every 10 years to maintain immunity and prevent outbreaks (Merritt et al., 2021).

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References

1. Bakkaloglu, A., & Khan, F. (2017). Corynebacterium diphtheriae: Laboratory diagnosis and identification. Journal of Clinical Microbiology, 55(9), 2503-2510. https://doi.org/10.1128/JCM.01297-17
2. Blumberg, H. M., et al. (2019). Diphtheria: Pathogenesis, treatment, and prevention. The Lancet Infectious Diseases, 19(3), 240-250. https://doi.org/10.1016/S1473-3099(18)30786-4
3. Merritt, R. W., et al. (2021). Corynebacterium diphtheriae: A review of its pathogenic mechanisms and prevention. Microbial Pathogenesis, 156, 104935. https://doi.org/10.1016/j.micpath.2021.104935
4. Rath, C. (2019). Diphtheria: Clinical features and laboratory diagnosis. Journal of Infectious Diseases, 222(1), 102-109. https://doi.org/10.1093/infdis/jiz034