Diseases of the Respiratory System: Microbiology Study Notes

Diseases of the Respiratory System

Overview of the Respiratory System

The respiratory system is responsible for gas exchange and is divided into the upper and lower respiratory tracts. Understanding its anatomy is essential for recognizing how various pathogens cause disease.

• Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx, and associated structures (e.g., tonsils, epiglottis).

• Lower Respiratory Tract: Comprises the larynx, trachea, bronchi, bronchioles, and lungs.

Key Point: The upper tract is more frequently exposed to pathogens, while infections of the lower tract are often more severe.

Upper Respiratory Tract Diseases

Bacterial Infections

• Epiglottitis: Inflammation of the epiglottis, often caused by Haemophilus influenzae type b. Can cause life-threatening airway obstruction. Treatment: Immediate antibiotics and airway management.

• Streptococcal Pharyngitis (Strep Throat): Caused by Streptococcus pyogenes. Symptoms: sore throat, fever, swollen lymph nodes. Treatment: Penicillin or other antibiotics.

• Scarlet Fever: Also due to S. pyogenes producing erythrogenic toxin. Symptoms: red rash, high fever, 'strawberry' tongue. Prevention: Prompt treatment of strep throat.

• Diphtheria: Caused by Corynebacterium diphtheriae. Symptoms: sore throat, pseudomembrane formation, difficulty breathing. Prevention: DTaP vaccine. Treatment: Antitoxin and antibiotics.

• Otitis Media: Middle ear infection, often following upper respiratory infections. Common agents: S. pneumoniae, H. influenzae. Treatment: Antibiotics; sometimes tympanostomy tubes.

Viral Infections

• Common Cold: Caused by rhinoviruses, coronaviruses, and others. Symptoms: runny nose, sneezing, sore throat. Treatment: Supportive care; no specific antiviral therapy.

Lower Respiratory Tract Diseases

Bacterial Infections

• Pertussis (Whooping Cough): Caused by Bordetella pertussis. Stages:

  • Catarrhal stage: Mild, cold-like symptoms.

  • Paroxysmal stage: Severe coughing fits with "whoop" sound.

  • Convalescent stage: Gradual recovery.

  Prevention: DTaP vaccine. Treatment: Macrolide antibiotics.

• Tuberculosis (TB): Caused by Mycobacterium tuberculosis.

  • Special Features: Cell wall contains mycolic acid (acid-fast), slow generation time (~20 hours).

  • Transmission: Airborne droplets.

  • Pathogenesis: Bacteria survive in macrophages, form granulomas (tubercles).

  • Diagnosis: Tuberculin skin test (Mantoux test) detects exposure to TB antigens.

  • Treatment Challenges: Long-term multi-drug therapy required; drug resistance is a major issue.

  • BCG Vaccine: Live attenuated vaccine; not routinely used in the US due to variable efficacy and interference with skin testing.

• Pneumonia: Inflammation of the lung alveoli, can be caused by various organisms.

  • Pneumococcal pneumonia: Streptococcus pneumoniae; most common type. Symptoms: high fever, productive cough, chest pain. Prevention: Pneumococcal vaccine.

  • Klebsiella pneumonia: Klebsiella pneumoniae; often in immunocompromised patients. Thick, bloody sputum.

  • Mycoplasmal pneumonia: Mycoplasma pneumoniae; "walking pneumonia," milder symptoms, common in young adults.

  • Haemophilus influenzae pneumonia: Often affects those with underlying conditions.

  • Legionellosis (Legionnaire’s Disease): Legionella pneumophila; associated with contaminated water sources (e.g., air conditioning systems).

Viral Infections

• Influenza (Flu): Caused by influenza viruses (types A, B, C).

  • Features: Enveloped RNA virus with hemagglutinin (HA) and neuraminidase (NA) spikes.

  • Hemagglutinin (HA): Binds to host cell receptors, facilitates entry.

  • Neuraminidase (NA): Assists in viral release from host cells.

  • Vaccine: Annual, due to antigenic drift and shift. Future research: Universal flu vaccine development.

• Viral Pneumonia: Can be caused by influenza, coronaviruses, and others.

  • SARS (Severe Acute Respiratory Syndrome): Caused by SARS-CoV.

  • MERS (Middle East Respiratory Syndrome): Caused by MERS-CoV.

  • COVID-19: Caused by SARS-CoV-2; symptoms range from mild to severe pneumonia.

• Respiratory Syncytial Virus (RSV): Major cause of lower respiratory tract infections in infants and young children.

Fungal Infections

• Histoplasmosis: Caused by Histoplasma capsulatum. Acquired by inhaling spores from soil, especially with bird or bat droppings.

• Coccidioidomycosis (Valley Fever): Caused by Coccidioides immitis. Endemic in southwestern US; inhalation of arthroconidia from soil.

• Pneumocystis Pneumonia: Caused by Pneumocystis jirovecii. Common in immunocompromised patients (e.g., AIDS).

• Blastomycosis: Caused by Blastomyces dermatitidis. Found in soil; can cause chronic pulmonary infection.

Summary Table: Major Respiratory Diseases

Disease	Pathogen	Key Symptoms	Treatment/Prevention
Streptococcal Pharyngitis	Streptococcus pyogenes	Sore throat, fever	Antibiotics
Diphtheria	Corynebacterium diphtheriae	Sore throat, pseudomembrane	DTaP vaccine, antitoxin
Pertussis	Bordetella pertussis	Severe cough, "whoop"	DTaP vaccine, antibiotics
Tuberculosis	Mycobacterium tuberculosis	Chronic cough, weight loss	Multi-drug therapy, BCG vaccine
Pneumococcal Pneumonia	Streptococcus pneumoniae	Fever, chest pain, cough	Antibiotics, vaccine
Influenza	Influenza virus	Fever, myalgia, cough	Antivirals, annual vaccine
COVID-19	SARS-CoV-2	Fever, cough, dyspnea	Supportive, vaccines
Histoplasmosis	Histoplasma capsulatum	Fever, cough, chest pain	Antifungals

Key Definitions

• Antigenic Drift: Minor changes in viral antigens due to mutations.

• Antigenic Shift: Major changes due to reassortment of viral genome segments.

• Granuloma: A mass of immune cells formed to wall off persistent pathogens (e.g., in TB).

• Pseudomembrane: A thick, gray membrane formed in the throat in diphtheria.

Example: Tuberculin Skin Test

The tuberculin skin test involves intradermal injection of purified protein derivative (PPD) from M. tuberculosis. A positive reaction (induration) indicates prior exposure or infection.

Example: Influenza Virus Structure

• Hemagglutinin (HA): Binds to sialic acid on host cells, mediates entry.

• Neuraminidase (NA): Cleaves sialic acid, facilitates viral release.

Relevant Equations

• Bacterial Generation Time: $N = N_0 \times 2^n$ Where $N$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Additional info: Some details (e.g., specific treatments, vaccine types) were expanded for academic completeness.