Microbial Diseases of the Respiratory System: Structure, Microbiome, and Bacterial Infections

Chapter 22: Microbial Diseases of the Respiratory System

Course Learning Objective

This chapter focuses on distinguishing infectious diseases of microbial origin in the respiratory system, including information about causative agents, signs and symptoms, diagnostic markers, treatment, and prevention.

Structures of the Respiratory System

Anatomy and Function

The respiratory system is responsible for gas exchange between the atmosphere and the blood. It is divided into two main parts:

• Upper respiratory system: Includes the nose, nasal cavity, pharynx, and associated structures such as the sinuses and ears.

• Lower respiratory system: Includes the larynx, trachea, bronchi, alveoli, and diaphragm.

Each part contains specialized structures and protective mechanisms to prevent infection.

Components of the Upper Respiratory System, Sinuses, and Ears

• Nose

• Nasal cavity

• Pharynx

• Uvula

• Sinuses and auditory tube (connects middle ear to pharynx)

• Various antimicrobial chemicals (e.g., lysozyme in mucus)

Components of the Lower Respiratory System

• Larynx

• Trachea

• Bronchi

• Alveoli

• Diaphragm

• Protective components: Ciliated mucous membrane, alveolar macrophages, and secretory antibodies (IgA)

Illustration of Respiratory Structures

The respiratory system includes interconnected passages and air sacs (alveoli) where gas exchange occurs. Alveolar macrophages patrol the alveoli to remove pathogens and debris.

Microbiome of the Respiratory System

Distribution and Role

• Lower respiratory system: Typically axenic (free of microorganisms) due to effective defense mechanisms.

• Upper respiratory system: Colonized by many microorganisms forming the normal microbiome.

• Normal microbiome limits the growth of pathogens by competitive exclusion and production of antimicrobial substances.

• Some members of the normal microbiome may act as opportunistic pathogens under certain conditions (e.g., immunosuppression).

Clinical Relevance

• MRSA (Methicillin-resistant Staphylococcus aureus) in normal microbiota can pose a risk to other patients in hospitals due to its resistance to antibiotics and potential for transmission.

Bacterial Diseases of the Upper Respiratory System, Sinuses, and Ears

Streptococcal Respiratory Diseases

Streptococcal infections are among the most common bacterial diseases affecting the upper respiratory tract.

• Pharyngitis: Sore throat and difficulty swallowing, often accompanied by fever, malaise, and headache.

• Laryngitis and bronchitis can occur if infection spreads to the lower respiratory tract.

• Complications: Scarlet fever, rheumatic fever, and acute glomerulonephritis may occur in some cases.

Pathogen and Virulence Factors

• Caused by group A streptococci (Streptococcus pyogenes).

• Virulence factors include:

  • M proteins: Interfere with phagocytosis and enhance adherence.

  • Hyaluronic acid capsule: Mimics host tissues, evading immune detection.

  • Streptokinases: Enzymes that break down blood clots, aiding spread.

  • C5a peptidase: Degrades complement protein C5a, reducing inflammation.

  • Pyrogenic toxins: Induce fever and rash.

  • Streptolysins: Lyse red and white blood cells.

Example: Streptococcal Pharyngitis ('Strep Throat')

• Common in children and adults.

• Rapid antigen detection tests and throat cultures are used for diagnosis.

• Treated with antibiotics such as penicillin.

• Prevention includes good hygiene and avoiding close contact with infected individuals.

Additional info: The notes above are expanded from brief points to provide academic context and definitions. The anatomical illustration referenced in the slides is described in text for clarity.