Microbial Diseases of the Respiratory System: Key Pathogens and Mechanisms

Microbial Diseases of the Respiratory System

Corynebacterium diphtheriae (Diphtheria)

Diphtheria is a serious bacterial infection affecting the respiratory tract, primarily caused by Corynebacterium diphtheriae. The disease is characterized by toxin production and the formation of a thick membrane in the throat.

• Type: Bacterium producing an exotoxin (primary virulence factor).

• Characteristics:

  • Toxin synthesis: Formation of a thick membrane at the site of infection.

  • Membrane composition: Immune cells and bacterial cells.

• Other features:

  • Can cause suffocation if untreated.

  • May cause heart and nerve damage.

• Prevention:

  • Immunization (DPT vaccine: Diphtheria-Pertussis-Tetanus).

• Notes:

  • Antitoxin and antibiotics (therapy) effective if administered early.

  • Antibody formation is gradual—presence indicates lack of early medical care.

Bordetella pertussis (Whooping Cough)

Whooping cough is a highly contagious respiratory disease caused by Bordetella pertussis. It is notable for its severe coughing fits and characteristic 'whoop' sound.

• Morphology: Gram-negative coccobacilli (short, rod-shaped).

• Virulence factors:

  • Capsule (protects against phagocytosis).

  • Exotoxins (damage respiratory tract).

• Pathogenesis:

  • Accumulation of thick mucus in respiratory tract.

  • Paroxysmal cough (severe, repetitive coughing fits).

  • Can cause brain damage due to oxygen deprivation.

• Prevention:

  • Immunization (DPT vaccine).

• Treatment:

  • Antibiotics (macrolides, e.g., erythromycin).

Mycobacterium tuberculosis (Tuberculosis)

Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis. It primarily affects the lungs but can spread to other organs.

• Type: Acid-fast bacterium (high lipid content in cell wall).

• Transmission: Aerosolized droplets; can remain suspended in air for several hours.

• Global impact: ~1 million deaths per year worldwide.

• Virulence factors:

  • Highly resistant and long-lived outside the host due to lipid content.

  • Can replicate and survive inside macrophages.

• Diagnosis:

  • Chest X-ray, sputum analysis, tuberculin skin test.

• Treatment:

  • Extended antibiotic therapy (months in duration).

  • Antimicrobial drugs: isoniazid, rifampin, ethambutol.

• Prevention:

  • Attenuated vaccine (BCG) available.

• Notes:

  • Exposure does not guarantee infection—depends on infectious dose ($D_{50}$).

  • Bacterium is resistant and persistent, making it a major global health concern.

Viral Infection: The Common Cold

The common cold is a mild viral infection of the upper respiratory tract, caused by several different viruses, most notably rhinoviruses.

• Primary viruses: Multiple—Rhinovirus is a major cause but not the only one.

• Key features:

  • Many different strains and viruses cause similar cold symptoms.

  • Symptoms: sneezing, sore throat, mild cough.

• Transmission:

  • Droplets (coughing, sneezing).

  • Pathogens can spread directly through air and indirectly via fomites.

• Notes:

  • Sneezing or coughing without covering mouth = aerosolized spread.

  • Touching contaminated surfaces transfers pathogens via mucous mistake.

Table: Comparison of Key Respiratory Pathogens

Pathogen	Type	Main Disease	Transmission	Prevention
Corynebacterium diphtheriae	Bacterium (exotoxin producer)	Diphtheria	Respiratory droplets	DPT vaccine
Bordetella pertussis	Bacterium (capsule, exotoxins)	Whooping cough	Respiratory droplets	DPT vaccine
Mycobacterium tuberculosis	Acid-fast bacterium	Tuberculosis	Aerosolized droplets	BCG vaccine
Rhinovirus (and others)	Virus	Common cold	Droplets, fomites	None (general hygiene)