Structure and Function of the Respiratory System

Overview of Respiratory Anatomy

The respiratory system is divided into upper and lower regions, each with distinct anatomical features and protective mechanisms against microbial invasion.

• Upper respiratory system: Includes the nose, pharynx, middle ear, and eustachian tubes. Saliva and tears help protect mucosal surfaces by flushing out pathogens and containing antimicrobial substances.

• Lower respiratory system: Comprises the larynx, trachea, bronchial tubes, and alveoli. The ciliary escalator moves particles toward the throat via ciliary action, while alveolar macrophages destroy microorganisms in the lungs. Respiratory mucus also protects mucosal surfaces.

Key Anatomical Features

• Cilia: Hair-like structures lining the respiratory tract that trap and move particles out of the airways.

• Alveoli: Tiny air sacs in the lungs where gas exchange occurs and where macrophages patrol for pathogens.

Normal Microbiota of the Respiratory System

Role of Microbiota

Normal microbiota play a crucial role in suppressing pathogenic microbes by competing for nutrients and producing inhibitory substances.

• Upper respiratory tract: Contains diverse microbiota that help prevent colonization by pathogens.

• Lower respiratory tract: Nearly sterile due to effective defense mechanisms such as ciliary action and immune cells.

Microbial Diseases of the Upper Respiratory System

Common Diseases and Symptoms

• Pharyngitis: Sore throat, often caused by viral or bacterial infection.

• Laryngitis: Inflammation of the larynx, leading to hoarseness or loss of voice.

• Tonsillitis: Inflammation of the tonsils, often with sore throat and difficulty swallowing.

• Sinusitis: Inflammation of the sinuses, usually self-limiting.

• Epiglottitis: Most life-threatening disease of the upper respiratory system; Haemophilus influenzae is the most common cause.

Bacterial Diseases of the Upper Respiratory System

Streptococcal Pharyngitis (Strep Throat)

• Caused by group A streptococci (GAS), Streptococcus pyogenes

• Gram-positive, resistant to phagocytosis

• Produces streptokinases (lyse clots) and streptolysins (cytotoxic)

• Symptoms: Local inflammation, fever, tonsillitis, enlarged lymph nodes

• Diagnosis: Rapid antigen detection tests, culturing for beta-hemolytic bacitracin-sensitive colonies

• Treatment: Penicillin

Scarlet Fever

• Caused by erythrogenic toxin produced by lysogenized S. pyogenes

• Often follows strep throat or skin infection

• Symptoms: High fever, pinkish red skin rash (sandpaper texture), strawberry-like tongue

• Treatment: To prevent rheumatic fever

Diphtheria

• Caused by Corynebacterium diphtheriae (Gram-positive bacillus, club-shaped)

• Forms tough grayish membrane in throat (fibrin and dead tissue), blocks air passage

• Exotoxin damages heart and kidneys

• Cutaneous diphtheria: Skin ulcers in adults

• Prevention: DTaP vaccine (diphtheria toxoid)

Otitis Media

• Infection of the middle ear, pus formation puts pressure on eardrum

• Causes: Streptococcus pneumoniae, nonencapsulated Haemophilus influenzae, Moraxella catarrhalis, S. pyogenes, respiratory syncytial viruses

• Common in children due to smaller auditory tube

• Treatment: Broad-spectrum penicillins

Viral Diseases of the Upper Respiratory System

The Common Cold

• Caused by over 200 different viruses, mainly rhinoviruses (30-50%) and coronaviruses (10-15%)

• Thrive in temperatures lower than body temperature

• Transmission: Aerosols, direct contact, fomites

• Symptoms: Sneezing, nasal secretion, congestion (about 1 week), can lead to laryngitis and otitis media, not accompanied by fever

• Treatment: Symptomatic (cough suppressants, antihistamines); antibiotics are ineffective

Lower Respiratory System Microbial Diseases

Major Diseases and Symptoms

• Bronchitis: Inflammation of bronchi, productive cough, wheezing, shortness of breath, fatigue, low fever

• Bronchiolitis: Inflammation of bronchioles, more severe than bronchitis, hypoxia, tachypnea

• Pneumonia: Inflammation of alveoli, filled with fluid/pus (consolidation), fever/chills, productive cough, chest pain, shortness of breath, tachypnea

Bacterial Diseases of the Lower Respiratory System

Pertussis (Whooping Cough)

• Caused by Bordetella pertussis (Gram-negative coccobacillus)

• Produces capsule for attachment to ciliated cells

• Tracheal cytotoxin: Damages ciliated cells, loss of ciliary escalator, mucus accumulates

• Pertussis toxin: A-B type exotoxin, causes systemic symptoms including hypoglycemia

• Stages:

  1. Catarrhal stage: Runny nose, mild cough, no/low fever, 1-2 weeks, highly contagious

  2. Paroxysmal stage: Violent coughing with gasping (whooping), communicable up to 3 weeks

  3. Convalescence stage: Subsiding cough, slow recovery, increased susceptibility to other infections

• Complications: Apnea, pneumonia, convulsions, encephalopathy

• Prevention: DTaP or Tdap vaccine (subunit)

• Treatment: Erythromycin or other macrolides (less effective after paroxysmal stage)

Tuberculosis

• Caused by Mycobacterium tuberculosis (acid-fast bacillus, obligate aerobe, slow-growing)

• Other causes: Mycobacterium bovis (bovine TB, Mycobacterium avium complex (late-stage HIV infection)

Pathogenesis

• Inhaled organisms phagocytized by alveolar macrophages; may survive and multiply

• Mycolic acids stimulate inflammatory response

• Bacteria isolated in tubercles; may heal and calcify (Ghon's complexes)

• Latent TB: Dormant bacilli

• Tubercle breakdown releases bacteria systemically (miliary TB)

Diagnosis

• Tuberculin skin test: Positive reaction indicates current or previous infection; T cells react with PPD

• Follow-up: X-ray/CT, acid-fast staining, culturing

• Rapid blood test for IFN-γ and NAAT: Higher specificity, less cross-reactivity

Treatment

• Minimum 6 months drug therapy (slow growth/dormancy)

• First-line drugs: isoniazid, rifampin, ethambutol, pyrazinamide

• Second-line drugs: Used if resistance develops; less effective, more toxic

• MDR strains: Resistant to first-line drugs

• XDR strains: Resistant to second-line drugs

Incidence and Vaccines

• 10 million cases annually, 2 million deaths

• Leading cause of death in HIV patients

• BCG vaccine: Live avirulent M. bovis; not widely used in US, can be fatal in HIV-infected children

Bacterial Pneumonias

• Typical pneumonia: Caused by S. pneumoniae

• Atypical pneumonia: Caused by other microbes

• Lobar pneumonia: Affects lung lobes

• Bronchopneumonia: Affects alveoli adjacent to lungs

• Pleurisy: Inflammation of pleural membranes

Pneumococcal Pneumonia

• Caused by S. pneumoniae (Gram-positive, encapsulated diplococci)

• Rapid onset, high fever, chest pain, rust-colored sputum

• Can spread to bloodstream, pleural cavity, meninges

• Treatment: Cephalosporins, macrolides

• Prevention: Conjugated pneumococcal vaccine

Haemophilus influenzae Pneumonia

• Gram-negative coccobacillus

• Symptoms resemble pneumococcal pneumonia, but usually slower onset, less fever/chest pain

• High risk: Children under 5, adults over 65

• Hib vaccine reduced incidence by 99% in children

• Diagnosis: Special media for X and V factors

• Treatment: Cephalosporins

Mycoplasmal Pneumonia

• Also called "walking pneumonia"

• Caused by Mycoplasma pneumoniae (no cell wall)

• Mild but persistent symptoms (3+ weeks): low fever, cough, headache, fatigue

• Common in children and young adults

• Treatment: Tetracyclines

Legionellosis

• Legionnaires' disease: First outbreak in 1976, deaths first attributed to viral pneumonia

• Caused by Legionella pneumophila (aerobic, gram-negative bacillus, forms biofilms)

• Grows in water/air conditioning systems, natural water, hospital water lines

• Transmission: Inhalation of aerosols (noncommunicable)

• Symptoms: Severe pneumonia, high fever

• Treatment: Azithromycin, macrolides

Psittacosis (Ornithosis)

• Caused by Chlamydia psittaci (gram-negative obligate intracellular bacterium)

• Elementary bodies from bird droppings transmitted through air

• Reservoir: Pet birds (parakeets, parrots), turkeys, poultry

• Symptoms: Pneumonia similar to typical pneumonia; subclinical infections common

• Treatment: Tetracyclines

Chlamydial Pneumonia

• Caused by Chlamydophila pneumoniae (gram-negative intracellular bacterium)

• Transmitted person-to-person (communicable)

• Mild respiratory illness, resembles mycoplasmal pneumonia

• Treatment: Azithromycin

Summary Table: Major Bacterial Pathogens of the Respiratory System

Key Terms and Concepts

• Pathogen: A microorganism that causes disease.

• Exotoxin: Toxin secreted by bacteria, often causing systemic effects.

• Biofilm: Community of microorganisms attached to a surface, often resistant to antibiotics.

• Obligate intracellular bacterium: Bacteria that must live inside host cells to survive and replicate.

• Antigenic drift/shift: Genetic changes in viral antigens, affecting immunity and vaccine effectiveness.

Additional info:

• Some content inferred from standard microbiology textbooks to provide complete explanations and context.

• Table entries and definitions expanded for clarity and academic completeness.