Pulmonary infections

Community-Acquired Pneumonia - The Usual Suspects

• Streptococcus pneumoniae: Most common cause of CAP. Gram-positive diplococci. Presents with rust-colored sputum.
• Haemophilus influenzae: Common in patients with COPD. Gram-negative coccobacillus.
• Moraxella catarrhalis: Often seen in the elderly and those with underlying lung disease. Gram-negative diplococcus.
• Staphylococcus aureus: Typically follows a viral illness, like influenza. Associated with cavitary lesions and empyema. Gram-positive cocci in clusters.

⭐ Rust-colored sputum is a classic, though not universally present, sign pointing towards S. pneumoniae infection.

Nosocomial Pneumonia - Hospital Horrors

• Pneumonia acquired ≥48 hours after hospital admission.
• Ventilator-associated pneumonia (VAP) is a major subtype, developing >48-72 hours after endotracheal intubation.
• Common Pathogens:
  • Gram-negative bacilli: Pseudomonas aeruginosa, E. coli, Klebsiella pneumoniae
  • Gram-positive cocci: Staphylococcus aureus (including MRSA)

⭐ Pseudomonas aeruginosa infection is a marker for severe, difficult-to-treat nosocomial pneumonia, often requiring multi-drug regimens.

Aspiration & Abscess - Wrong Pipe Woes

• Risk Factors: Altered consciousness (↓gag reflex via alcohol, seizures, anesthesia), dysphagia, poor dentition.
• Pathophysiology: Aspiration of oropharyngeal contents → pneumonitis → necrosis & abscess formation (~1-2 weeks).
  • Microbes: Polymicrobial; oral anaerobes (Peptostreptococcus, Fusobacterium, Bacteroides) + aerobes.
  • Features: Foul-smelling sputum, fever, weight loss.
• Imaging: CXR/CT reveals a cavitary lesion with an air-fluid level.

⭐ The most common location for an aspiration abscess is the posterior segment of the right upper lobe (if supine) or the basal segment of the right lower lobe (if upright/seated).

Tuberculosis - The Great White Plague

• Etiology: Mycobacterium tuberculosis (acid-fast bacillus), transmitted via inhalation.
• Pathogenesis: Formation of caseating granulomas (central necrosis, Langhans giant cells).
• Primary TB: Subpleural Ghon focus + hilar lymph node involvement → Ghon complex. Usually becomes latent.
• Secondary TB: Reactivation, often due to immunosuppression, with a predilection for lung apices.

⭐ Secondary TB favors the lung apices due to high oxygen tension (↑ V/Q ratio), ideal for the aerobic M. tuberculosis.

Fungal & Opportunistic - Fungal Fiends & Freeloaders

• Aspergillus fumigatus: Allergic (ABPA), cavity-filling (Aspergilloma), or invasive (in immunocompromised).
  • Septate hyphae with 45° branching.
  • 📌 Mnemonic: A for Acute Angle.
• Pneumocystis jirovecii (PJP): Atypical fungus causing diffuse interstitial pneumonia.
  • Risk: CD4 < 200 cells/mm³.
  • Dx: "Crushed ping-pong ball" cysts on silver stain.
• Cryptococcus neoformans: Encapsulated yeast from pigeon droppings.
  • Dx: India ink stain shows halos.

⭐ Invasive aspergillosis in neutropenic patients classically shows a "halo sign" on CT (hemorrhage around a nodule), which can later form an "air crescent sign" during recovery.

• S. pneumoniae is the leading cause of Community-Acquired Pneumonia (CAP); Mycoplasma is classic in young adults.
• Hospital-Acquired (HAP) and Ventilator-Associated (VAP) pneumonias are frequently caused by Pseudomonas and MRSA.
• Aspiration pneumonia typically involves oral anaerobes and classically localizes to the right lower lobe.
• Atypical pneumonias (Mycoplasma, Legionella) present insidiously with interstitial infiltrates on chest X-ray.
• Reactivated Tuberculosis characteristically features upper lobe cavitary lesions and constitutional symptoms.
• Pneumocystis jirovecii (PJP) is a key opportunistic pneumonia in HIV patients with a CD4 count < 200.