Bacterial Pathogens: Corynebacterium diphtheriae and Mycobacterium tuberculosis

Corynebacterium diphtheriae

General Characteristics

Corynebacterium diphtheriae is a Gram-positive, irregular rod-shaped bacterium known for causing diphtheria. It exhibits pleomorphism, meaning it can appear in various shapes such as straight, curved, club-shaped, filamentous, or swollen forms. The bacterium often arranges in palisades (side-by-side alignment).

• Gram-positive: Retains crystal violet stain due to thick peptidoglycan cell wall.

• Pleomorphism: Variable shapes and sizes.

• Reservoir: Healthy carriers can harbor the bacterium, maintaining a potential for diphtheria outbreaks.

• Transmission: Acquired via respiratory droplets from carriers or infected individuals.

Example: Outbreaks can occur in populations with low vaccination rates or close contact settings.

Pathology of Diphtheria

Diphtheria progresses in two main stages: local infection and toxemia.

• Local Infection: Primarily affects the upper respiratory tract, causing inflammation.

  • Sore throat, nausea, vomiting, swollen lymph nodes.

  • Production of an exotoxin that inhibits protein synthesis and kills cells.

  • Formation of a pseudomembrane in the throat, which can obstruct airways.

  • Cutaneous diphtheria may occur as a secondary infection affecting the skin.

• Toxemia: The diphtheria toxin enters the bloodstream, targeting organs such as the heart and nerves.

Example: Severe cases may result in myocarditis or neurological complications due to toxin spread.

Diagnostic Methods for Corynebacterium diphtheriae

Diagnosis involves clinical observation and laboratory tests.

• Clinical signs: Presence of pseudomembrane and throat swelling.

• Staining: Microscopic examination after Gram or special stains.

• Serological assay: Detection of antibodies or toxins.

Treatment and Prevention

• Diphtheria antitoxin: Neutralizes circulating toxin.

• Antibiotics: Penicillin or erythromycin to eliminate bacteria.

• Prevention: Toxoid vaccine series and regular boosters.

Example: The DTaP vaccine protects against diphtheria, tetanus, and pertussis.

Mycobacteria: Acid-Fast Bacilli

General Characteristics

Mycobacteria are Gram-positive, irregular bacilli notable for their acid-fast properties due to a waxy cell wall rich in mycolic acids.

• Acid-fast staining: Retain dye after acid-alcohol wash (Ziehl-Neelsen stain).

• Strict aerobes: Require oxygen for growth.

• Produce catalase: Enzyme that breaks down hydrogen peroxide.

• No capsules, flagella, or spores.

• Slow growth rate.

Example: Mycobacterium tuberculosis is the causative agent of tuberculosis.

Mycobacterium tuberculosis: Virulence and Pathogenesis

Mycobacterium tuberculosis does not produce exotoxins or enzymes that directly cause disease. Its virulence is attributed to complex waxes and the cord factor, which prevent destruction by lysosomes or macrophages.

• Virulence factors: Mycolic acids and cord factor protect against immune defenses.

• Intracellular survival: Can multiply within macrophages.

Epidemiology and Transmission of Tuberculosis

Tuberculosis (TB) is influenced by host factors and environmental conditions.

• Risk factors: Malnutrition, compromised immune system, poor access to medical care, lung damage, genetics.

• Transmission: Airborne respiratory droplets.

• High-risk populations: Recent immigrants, immunocompromised individuals.

Example: TB outbreaks are more common in crowded or resource-limited settings.

Course of Infection and Disease

Only 5% to 10% of infected individuals develop clinical disease. TB progresses slowly and is usually contained in the lungs.

• Primary tuberculosis: Initial infection, often asymptomatic or mild.

• Secondary tuberculosis: Reactivation or reinfection, more severe symptoms.

• Disseminated (extrapulmonary) tuberculosis: Spread to other organs (lymph nodes, kidneys, bones, genital tract, brain, meninges).

Initial Infection and Primary TB

After inhalation, M. tuberculosis is phagocytosed by macrophages and multiplies intracellularly. The immune system responds after 3-4 weeks, forming tubercles (granulomas).

• Tubercles: Necrotic, caseous lesions that may heal and calcify.

Latent and Recurrent TB

If the patient does not recover from primary TB, reactivation can occur. Tubercles expand into the bronchial tubes and upper respiratory tract, causing severe symptoms.

• Symptoms: Violent coughing, greenish or bloody sputum, fever, anorexia, weight loss, fatigue.

• Mortality: Untreated, up to 60% mortality rate.

Extrapulmonary TB

TB can spread beyond the lungs to regional lymph nodes, kidneys, long bones, genital tract, brain, and meninges.

• Complications: May cause organ-specific symptoms and damage.

Table: Comparison of Corynebacterium diphtheriae and Mycobacterium tuberculosis

Feature	Corynebacterium diphtheriae	Mycobacterium tuberculosis
Gram Stain	Positive	Positive
Shape	Irregular rod, club-shaped, pleomorphic	Irregular bacilli, acid-fast
Transmission	Respiratory droplets	Airborne respiratory droplets
Main Disease	Diphtheria	Tuberculosis
Virulence Factor	Exotoxin (inhibits protein synthesis)	Mycolic acids, cord factor
Prevention	Toxoid vaccine	BCG vaccine (not mentioned in notes, inferred)

Additional info: BCG vaccine is used for TB prevention in many countries.