Respiratory Pathogens and Clinical Manifestations

• Measles: A highly contagious virus that presents with the "3Cs" (cough, coryza, and conjunctivitis) and Koplik spots on the oral mucosa.

Other Viral Pathogens

• Coronaviruses: These are enveloped +ssRNA viruses associated with the common cold and more severe diseases like SARS.
• Rhinovirus: A +ssRNA virus that is a common cause of the cold; it typically binds to the ICAM-1 receptor.

Bacterial Pathogens

Bacterial respiratory infections are broadly divided into "typical" (often involving the lung parenchyma with sudden onset) and "atypical" (often presenting with milder, interstitial symptoms).

Aspiration Syndromes and Lung Abscesses

Aspiration occurs when foreign material enters the respiratory tract, often due to altered consciousness (e.g., stroke, alcoholism) or dysphagia.

1. Aspiration Pneumonitis: This is a chemical injury caused by the aspiration of acidic gastric contents. It presents abruptly, within hours of the event, and is managed with supportive care rather than antibiotics.
2. Aspiration Pneumonia: This is an actual infection of the lung parenchyma caused by aspirated oropharyngeal organisms (such as Peptostreptococcus and Fusobacterium). It develops over days and often involves the right lower lobe (RLL) because the right bronchus is wider, shorter, and more vertical.
3. Lung Abscesses: These are complications of necrotizing pneumonia. They present with weeks of fever, weight loss, and the production of foul-smelling sputum. On a chest X-ray, they appear as cavitary lesions with an air-fluid level.

Typical Bacterial Pathogens

• Streptococcus pneumoniae: A Gram-positive, lancet-shaped diplococcus. Its primary virulence factor is a polysaccharide capsule that prevents phagocytosis. It causes lobar pneumonia, meningitis, and otitis media.
• Haemophilus influenzae: A Gram-negative coccobacillus. The encapsulated Type b (Hib) strain can cause epiglottitis and meningitis and is prevented by the PRP vaccine. Nontypeable strains are nonencapsulated and typically cause otitis media or sinusitis.
• Staphylococcus aureus: Often causes severe, necrotizing pneumonia, particularly in young adults following an influenza infection.
• Corynebacterium diphtheriae: Causes pseudomembranous pharyngitis (a gray, coalescing membrane on the throat). Its toxin ADP-ribosylates EF-2, inhibiting protein synthesis, which can lead to systemic effects like myocarditis and neurologic toxicity. It is prevented by the DPT vaccine.
• Bordetella pertussis: Causes whooping cough (paroxysmal cough). Its toxin ADP-ribosylates $G_i$, which increases cAMP levels, interfering with leukocyte recruitment and phagocytosis.

Atypical Bacterial Pathogens

Atypical pathogens often lack a peptidoglycan cell wall, making them resistant to beta-lactam antibiotics.

• Mycoplasma pneumoniae: Lacks a cell wall and causes "walking pneumonia". It can trigger cold agglutinin disease, a Type II hypersensitivity where IgM anti-I antibodies react with red blood cells, causing hemolysis.
• Legionella pneumophila: An intracellular organism that causes pneumonia often accompanied by diarrhea and hyponatremia.
• Chlamydia pneumoniae: An obligate intracellular organism with an atypical cell wall that requires macrolides or tetracyclines for treatment.

Specialized Clinical Contexts

Cystic Fibrosis (CF)

In patients with Cystic Fibrosis, impaired mucociliary clearance leads to chronic infections. Pseudomonas aeruginosa is a common pathogen in these patients; it produces a blue-green pigment and Exotoxin A, which, like the diphtheria toxin, inhibits EF-2.

Summary of Toxin Mechanisms

Exam Tip: Distinguishing Toxins

• Diphtheria/Pseudomonas Exotoxin A: Targets EF-2 (inhibits protein synthesis).
• Pertussis Toxin: Targets $G_i$ (increases cAMP).
• Cholera Toxin: Targets $G_s$ (increases cAMP).