Community-Acquired Pneumonia (CAP)
Definition: pneumonia acquired outside hospital or within 48h of admission. Most common organism overall: Streptococcus pneumoniae (pneumococcus) — lancet-shaped diplococci, Gram+; presents with sudden fever, chills, productive cough, lobar consolidation, rust-colored sputum, + herpes labialis; vaccinate asplenic/elderly. Other CAP organisms by clinical clue: Haemophilus influenzae — COPD patients; Moraxella catarrhalis — COPD, elderly, encapsulated. Gram-negative CAP: Klebsiella pneumoniae — alcoholics + diabetics; 'currant jelly sputum' (blood + mucus), upper lobe cavitation; most common Gram-negative CAP. Staphylococcus aureus — post-influenza pneumonia, cavitary lesions, pneumatoceles; IV drug users; MRSA in healthcare-exposed. Pseudomonas aeruginosa — cystic fibrosis, bronchiectasis, immunocompromised, prolonged hospitalization.
Atypical Pneumonia Organisms
Atypical pneumonia = 'walking pneumonia' — milder presentation, no lobar consolidation, poorly visible organisms on Gram stain (not cell-wall dependent), lack response to beta-lactams. (1) Mycoplasma pneumoniae: most common atypical; young adults/college students; insidious onset, dry cough, headache, arthralgias; CXR patchy infiltrate out of proportion to mild symptoms; cold agglutinins (IgM against RBC I antigen) → can cause hemolytic anemia; treat with azithromycin or doxycycline. (2) Legionella pneumophila: water reservoir (cooling towers, hospital water, hot tubs); outbreaks in hotels/hospitals; presents with high fever + GI symptoms (diarrhea, nausea) + neurologic symptoms + hyponatremia (SIADH) — 'Legionnaire's disease'; urine antigen is best rapid test; NOT transmitted person-to-person; treat with fluoroquinolone or azithromycin. (3) Chlamydophila (Chlamydia) pneumoniae: young adults, slowly progressive; dry cough. (4) Chlamydia psittaci: bird/parrot exposure (psittacosis) — treat with doxycycline. (5) Coxiella burnetii: Q fever — cattle/sheep/goats exposure; does NOT require arthropod vector; treat with doxycycline.
Hospital-Acquired Pneumonia (HAP) and VAP
HAP: pneumonia occurring ≥48 hours after hospital admission. VAP (ventilator-associated pneumonia): HAP in intubated patients ≥48h after intubation. Pathogens: Gram-negative bacilli predominate — Pseudomonas aeruginosa, Klebsiella, E. coli, Acinetobacter (common in ICUs, can be multidrug-resistant). Gram-positive: MRSA (15-20% of HAP). Treatment: broad-spectrum coverage including antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, or carbapenem) ± vancomycin/linezolid for MRSA coverage if risk factors present. Risk factors for MRSA: prior MRSA infection, prior IV antibiotics, hemodialysis, skin/wound infection.
Pneumonia in Special Populations
HIV/immunocompromised (CD4 <200): Pneumocystis jirovecii pneumonia (PCP) — most common life-threatening OI in AIDS. Presents with insidious dyspnea, dry cough, fever; CD4 <200; CXR bilateral diffuse interstitial (ground-glass) infiltrates; elevated LDH; diagnose with BAL silver stain or direct fluorescent antibody; treat with TMP-SMX (first-line); prophylaxis with TMP-SMX when CD4 <200. Aspiration pneumonia: alcoholics, stroke, altered sensorium; right lower lobe most common (right mainstem bronchus more vertical); polymicrobial including oral anaerobes — clindamycin or amoxicillin-clavulanate. Neonates (0-3 months): Group B Streptococcus (#1 neonatal pneumonia), E. coli, Listeria — treat with ampicillin + gentamicin. Elderly/nursing home: S. pneumoniae + Gram-negatives ± aspiration — use CAP outpatient criteria (CURB-65 or PSI score) to determine admission.