Microbial Diseases of the Respiratory System

Introduction

The respiratory system is a frequent site of microbial infection due to constant exposure to airborne pathogens. Understanding the interplay between microbial virulence, host defenses, and environmental factors is essential for predicting, preventing, and managing respiratory diseases.

Normal Respiratory Defenses

Anatomy and Microbiome

• Upper respiratory tract: Includes the nose, nasal cavity, sinuses, and pharynx. Densely colonized by normal microbiota.

• Lower respiratory tract: Comprises the larynx, trachea, bronchi, bronchioles, and lungs. Traditionally described as axenic (free of microorganisms) due to effective clearance mechanisms.

• Modern view: Healthy lungs may contain a low-biomass microbiome, but the axenic model is useful for teaching.

Host Defenses and Their Weakening

• Colonization resistance: Normal microbiota compete with pathogens for space and nutrients, and may produce inhibitory substances.

• Opportunistic infections: Normal residents (e.g., Streptococcus pneumoniae) can cause disease if defenses are compromised.

Pathogenesis and Transmission

Mechanisms of Disease

Transmission Routes and Prevention

• Symptoms often overlap due to shared inflammatory pathways (e.g., fever, cough, congestion).

Bacterial Diseases of the Respiratory System

Streptococcal Diseases

• Organism: Streptococcus pyogenes (Group A strep), S. pneumoniae

• Transmission: Droplets, direct contact

• Virulence: Adhesins, M protein, exotoxins

• Symptoms: Sore throat, fever, tonsillar exudate, scarlet fever rash

• Complications: Rheumatic fever, glomerulonephritis

• Prevention: Hygiene, prompt treatment, no routine vaccine for Group A strep

Diphtheria

• Organism: Corynebacterium diphtheriae (toxin gene via bacteriophage)

• Transmission: Droplets, contact with lesions

• Virulence: Diphtheria toxin inhibits protein synthesis

• Symptoms: Sore throat, gray pseudomembrane, neck swelling

• Prevention: Diphtheria toxoid vaccine, boosters

Sinusitis, Otitis Media, and Pneumonia

• Common organisms: S. pneumoniae, Haemophilus influenzae, Moraxella catarrhalis

• Pathogenesis: Viral infection/allergy blocks drainage, allowing bacterial growth

• Symptoms: Sinus congestion, facial pain, ear pain, fever

• Prevention: Hygiene, immunization, reduce smoke exposure

Major Causes of Bacterial Pneumonia

Legionella and Tuberculosis

• Legionella pneumophila: Inhaled water aerosols, intracellular survival, pneumonia with possible GI and CNS symptoms. Prevention: water system control.

• Mycobacterium tuberculosis: Airborne, survives in macrophages, granuloma formation, latent vs. active disease. Prevention: case finding, isolation, BCG vaccine in some countries.

Pertussis and Anthrax

• Pertussis (Bordetella pertussis): Droplet spread, adhesins and toxins damage cilia, prolonged cough, infants at high risk. Prevention: DTaP/Tdap vaccination.

• Inhalation anthrax (Bacillus anthracis): Inhaled spores, toxin-mediated mediastinal disease, not person-to-person. Prevention: occupational controls, vaccine for high-risk groups.

Viral Diseases of the Respiratory System

Common Cold

• Organisms: Rhinoviruses, seasonal coronaviruses, others

• Transmission: Droplets, hand-to-mucosa, surfaces

• Symptoms: Sneezing, runny nose, sore throat, mild cough

• Prevention: Hygiene, air quality, no universal vaccine

Influenza

• Organisms: Influenza A and B (enveloped, segmented RNA viruses)

• Transmission: Droplets, aerosols, hands

• Virulence: Hemagglutinin (attachment), neuraminidase (release)

• Symptoms: Abrupt fever, myalgia, cough, fatigue

• Prevention: Annual vaccination, antivirals, hygiene

• Antigenic drift: Gradual change; Antigenic shift: Major reassortment (pandemic potential)

Coronavirus Syndromes

• Organisms: SARS-CoV, MERS-CoV, SARS-CoV-2, seasonal coronaviruses

• Transmission: Droplets, airborne (especially SARS-CoV-2), animal reservoirs (MERS)

• Symptoms: Fever, cough, dyspnea, fatigue, GI symptoms possible

• Prevention: Vaccination (COVID-19), PPE, air quality

RSV and Hantavirus

• RSV (Respiratory Syncytial Virus): Enveloped RNA virus, fusion protein causes syncytia, bronchiolitis in infants, spread by secretions and surfaces. Prevention: hygiene, vaccines/antibodies for high-risk groups.

• Hantavirus: Enveloped segmented RNA virus, inhaled rodent excreta, causes pulmonary capillary leak and edema. Prevention: rodent control, safe cleanup.

Comparison Table: Viral Disease Patterns

Fungal Diseases of the Respiratory System

Coccidioidomycosis

• Organisms: Coccidioides immitis, C. posadasii (dimorphic fungi)

• Transmission: Inhalation of arthroconidia from dry soil

• Pathogenesis: Spherules with endospores in tissue

• Symptoms: Often asymptomatic; Valley fever: cough, fever, chest pain, erythema nodosum

• Prevention: Dust control, PPE, no routine vaccine

Histoplasmosis

• Organism: Histoplasma capsulatum (dimorphic fungus)

• Transmission: Inhalation of spores from bird/bat enriched soil

• Pathogenesis: Yeasts survive in macrophages, granuloma formation

• Symptoms: Often mild; severe in immunocompromised

• Prevention: Avoid contaminated dust, PPE, no routine vaccine

Pneumocystis Pneumonia (PCP)

• Organism: Pneumocystis jirovecii (fungus)

• Transmission: Airborne, person-to-person

• Risk: Immunocompromised (e.g., advanced HIV, transplant)

• Symptoms: Fever, dry cough, hypoxemia, diffuse lung infiltrates

• Prevention: HIV treatment, prophylaxis for high-risk

Applying Disease Patterns

Predictive Questions

1. What reached the airway? (e.g., human pathogen, water aerosol, soil spore, rodent dust)

2. Which defense failed? (e.g., mucus clearance, immunity)

3. What causes injury? (e.g., toxin, inflammation)

4. Which prevention step interrupts the chain?

High-Yield Contrasts

• TB vs. latent TB: Active is contagious; latent is not.

• Legionella vs. pneumococcus: Water system exposure vs. upper tract colonization.

• Diphtheria vs. pertussis: Toxin pseudomembrane vs. ciliary damage and cough.

• Histoplasma vs. Coccidioides: Bird/bat soil and yeast in macrophages vs. arid soil and spherules.

• PCP vs. environmental fungi: Opportunistic, airborne, person-to-person vs. soil exposure.

Practice Questions (Selected)

1. Why is the healthy lower tract described as axenic in introductory texts? Answer: Defenses continually clear microbes.

2. How can upper tract residents contribute to otitis media? Answer: They grow after drainage is blocked.

3. A gray adherent throat membrane most strongly suggests which mechanism? Answer: Diphtheria toxin induced cell death.

4. Pneumonia after contaminated water mist exposure suggests which organism? Answer: Legionella pneumophila.

5. Which statement about latent TB is correct? Answer: It has no symptoms and is not contagious.

Key Definitions

• Axenic: Free of microorganisms.

• Colonization: Presence of organisms without disease.

• Infection: Organisms establish and multiply in the host.

• Infectious disease: Infection produces damage or dysfunction.

• Granuloma: Organized collection of immune cells, often containing infection.

• Antigenic drift: Gradual genetic change in viruses.

• Antigenic shift: Major genetic reassortment, especially in influenza A.

Summary Table: Major Respiratory Pathogens

Additional info:

• For equations, note that the study guide does not provide specific formulas, but relevant equations in respiratory microbiology may include calculations for infection rates, basic reproduction number (), or oxygen diffusion, which can be referenced as needed.

• For more detailed clinical management, always refer to current public health and facility guidelines.