Respiratory System Infections: Microbiology Principles and Pathogens

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Respiratory System Infections

Overview of the Respiratory Tract

The respiratory tract is divided into the upper and lower regions, each with distinct anatomical structures and microbial communities. Understanding these divisions is essential for recognizing how infections develop and spread.

Upper respiratory tract: Includes the paranasal sinuses, nasal cavity, pharynx (throat), and larynx (voice box).
Lower respiratory tract: Comprises the trachea (windpipe), bronchi, bronchioles, and alveoli within the lungs.
Alveoli: Tiny air sacs where gas exchange occurs; highly susceptible to infection due to their delicate structure.

Diagram of the human respiratory tract, labeling upper and lower regions

Normal Microbiota of the Respiratory Tract

The respiratory tract is colonized by diverse microbial communities that play a role in health and disease. The composition varies by anatomical site.

Nasal cavity: Dominated by Staphylococcus and Streptococcus species, along with Actinobacteria and Proteobacteria.
Pharynx (throat): Contains Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, and Fusobacteria. Common genera include Prevotella, Capnocytophaga, Campylobacter, Veillonella, Streptococcus, Neisseria, and Haemophilus.
Lungs: Main genera are Prevotella, Veillonella, Streptococcus, Neisseria, Haemophilus, and Fusobacterium.

Diagram showing the distribution of normal microbiota in the respiratory tract

Host Defenses in the Respiratory Tract

Mucociliary Escalator

The mucociliary escalator is a critical defense mechanism that traps and removes microbes and debris from the respiratory tract.

Mucus-producing cells: Secrete mucus that traps pathogens.
Ciliated epithelial cells: Move mucus and trapped particles upward toward the mouth for expulsion.

Diagram of the mucociliary escalator in the lower respiratory tract

Sinus Health and Sinusitis

Healthy sinuses are lined with ciliated mucus membranes that help clear microbes. Inflammation and blockage can lead to sinusitis, allowing microbes to accumulate and cause infection.

Sinusitis: Inflammation of the sinuses, often due to infection or allergies, resulting in blocked mucus drainage.

Diagram comparing healthy sinuses and sinusitis

Common Respiratory Pathogens

Viral Pathogens

Respiratory Syncytial Virus (RSV): A major cause of lower respiratory tract infections in infants and young children.
Adenovirus: Causes a range of illnesses from the common cold to pneumonia.
Influenza virus: Responsible for seasonal flu epidemics; can cause severe respiratory illness.
SARS-CoV-2: The causative agent of COVID-19, leading to a spectrum of respiratory symptoms.

RSV virions under electron microscopy Adenovirus virions under electron microscopy Influenza virion under electron microscopy SARS-CoV-2 virions under electron microscopy

Bacterial Pathogens

Streptococcus pneumoniae: Leading cause of bacterial pneumonia and otitis media.
Bordetella pertussis: Causes whooping cough, especially severe in infants and young children.
Mycobacterium tuberculosis: Agent of tuberculosis, a chronic lung infection.
Mycoplasma pneumoniae: Causes atypical (walking) pneumonia.
Legionella pneumophila: Responsible for Legionnaires' disease, a severe form of pneumonia.
Francisella tularensis: Causes tularemia, which can involve the lungs.

Streptococcus pneumoniae under electron microscopy Bordetella pertussis attached to cilia Mycobacterium tuberculosis (pink cells) under microscopy Mycoplasma pneumoniae under electron microscopy Legionella pneumophila under electron microscopy Francisella tularensis under electron microscopy

Fungal Pathogens

Blastomyces dermatitidis: Causes blastomycosis, endemic in certain U.S. regions.
Coccidioides species: Cause coccidioidomycosis (Valley fever), endemic in the southwestern U.S.
Histoplasma capsulatum: Causes histoplasmosis, endemic in the Ohio and Mississippi River valleys.
Aspergillus species: Cause aspergillosis, especially in immunocompromised hosts.
Rhizopus species: Cause mucormycosis, a rare but serious infection.
Pneumocystis jirovecii: Causes pneumonia in immunocompromised individuals.

Blastomyces dermatitidis under microscopy Coccidioides species under microscopy Histoplasma capsulatum under microscopy Aspergillus species under microscopy Rhizopus species under microscopy Pneumocystis jirovecii under microscopy

Selected Respiratory Infections and Their Features

Otitis Media (Middle Ear Infection)

Otitis media is a common bacterial infection of the middle ear, especially in children due to anatomical differences in the Eustachian tube.

Symptoms: Ear pain, fever, irritability.
Prevention: Tympanostomy tubes may be inserted to reduce infection risk in recurrent cases.

Diagram of child and adult ear anatomy and tympanostomy tube

Streptococcal Pharyngitis (Strep Throat)

Strep throat is caused by Streptococcus pyogenes and is characterized by sore throat, fever, and swollen lymph nodes. Complications can include scarlet fever and rheumatic fever.

Symptoms: Red, swollen throat with white patches, fever, headache.
Complications: Scarlet fever (rash, "strawberry tongue"), rheumatic fever (autoimmune response).

Clinical image of strep throat Scarlet fever rash and strawberry tongue Diagram of S. pyogenes M protein and cross-reactive antibodies

Pertussis (Whooping Cough)

Pertussis is a highly contagious respiratory disease caused by Bordetella pertussis. It is most severe in infants and young children.

Symptoms: Severe coughing fits, inspiratory "whoop," post-tussive vomiting.
Prevention: Vaccination (DTaP/Tdap) is effective in reducing incidence.

Graph of pertussis incidence by age group Bordetella pertussis attached to cilia

Tuberculosis (TB)

Tuberculosis is a chronic lung infection caused by Mycobacterium tuberculosis. It can exist in latent or active forms and is diagnosed using the tuberculin skin test.

Pathogenesis: Inhaled bacteria are phagocytosed by macrophages; granulomas form to contain infection.
Diagnosis: Tuberculin skin test (Mantoux test) measures induration after injection of purified protein derivative (PPD).
Interpretation: Induration size thresholds vary by risk group (e.g., ≥5 mm for immunocompromised, ≥10 mm for high-risk, ≥15 mm for low-risk individuals).

Diagram of TB infection stages: initial, latent, active Administering the tuberculin skin test Interpretation of tuberculin skin test results Mycobacterium tuberculosis under microscopy

Fungal Lung Infections

Several fungi can cause respiratory infections, especially in immunocompromised hosts or in certain geographic regions.

Blastomycosis: Caused by Blastomyces dermatitidis; spores are inhaled and can disseminate.
Coccidioidomycosis: Caused by Coccidioides species; inhaled arthroconidia develop into spherules in the lungs.
Histoplasmosis: Caused by Histoplasma capsulatum; spores are inhaled, and yeast forms colonize the lungs.

Blastomycosis infection cycle and endemic regions Coccidioidomycosis infection cycle and endemic regions Histoplasmosis infection cycle and endemic regions

Summary Table: Key Respiratory Pathogens and Diseases

Pathogen	Type	Disease(s)	Key Features
Streptococcus pneumoniae	Bacterium	Pneumonia, Otitis media	Gram-positive cocci, capsule, common in children
Bordetella pertussis	Bacterium	Pertussis (whooping cough)	Gram-negative coccobacillus, toxin-mediated disease
Mycobacterium tuberculosis	Bacterium	Tuberculosis	Acid-fast bacillus, granuloma formation
Influenza virus	Virus	Influenza (flu)	Segmented RNA genome, antigenic drift/shift
RSV	Virus	Bronchiolitis, pneumonia (infants)	Enveloped RNA virus, syncytia formation
Blastomyces dermatitidis	Fungus	Blastomycosis	Dimorphic, endemic in certain U.S. regions
Histoplasma capsulatum	Fungus	Histoplasmosis	Dimorphic, found in soil with bird/bat droppings

Additional info:

Respiratory infections can be caused by bacteria, viruses, or fungi, and their epidemiology often depends on host factors and geographic location.
Vaccination and public health measures are critical in controlling the spread of many respiratory pathogens.