Community-acquired pneumonia (CAP) is one of those “Step 1 meets real life” topics: you’ll see it in vignettes constantly because it forces you to integrate microbiology, immunology, physiology, and pharmacology. The good news is CAP is highly pattern-based—if you learn the big organisms, classic presentations, and first-line treatments (plus a few Step-favorite traps), you can crush most questions quickly.
What counts as Community-Acquired Pneumonia?
Definition (CAP): An acute infection of the lung parenchyma acquired outside the hospital (or diagnosed within ~48 hours of admission) in a patient not recently in a healthcare facility.
Why Step cares: The setting strongly predicts the pathogens and therefore empiric antibiotics.
CAP vs. other pneumonias (quick sorting)
| Type | Classic context | Pathogens (Step vibe) |
|---|---|---|
| CAP | Normal community setting | S. pneumoniae, H. influenzae, atypicals (Mycoplasma, Chlamydophila, Legionella), respiratory viruses |
| Aspiration pneumonia | Altered consciousness, dysphagia | Oral anaerobes (e.g., Bacteroides, Fusobacterium, Peptostreptococcus) |
| Hospital-acquired/VAP | ≥48h after admission or ventilator | S. aureus (incl MRSA), Pseudomonas, Gram-negative rods |
Pathophysiology (what’s actually happening in the lung)
How infection establishes itself
CAP usually starts with microaspiration of oropharyngeal flora during sleep. Normally, defenses clear it:
- Mucociliary escalator
- Alveolar macrophages
- IgA (upper airway) and IgG (lower airway)
- Neutrophil recruitment (especially important for encapsulated bacteria)
When defenses are impaired (viral infection, smoking, COPD, alcoholism, immunosuppression, extremes of age), pathogens reach alveoli and trigger inflammatory exudate.
Why oxygenation drops
Inflammatory exudate fills alveoli → ventilation decreases in affected regions while perfusion persists → V/Q mismatch (shunt physiology) → hypoxemia.
This is why pneumonia can cause low PaO₂ and increased work of breathing.
Lobar vs bronchopneumonia patterns (high yield)
- Lobar pneumonia: confluent consolidation of a lobe (classically pneumococcus)
- Bronchopneumonia: patchy, multilobar infiltrates (often staph, gram-negatives)
- Interstitial pneumonia: diffuse interstitial infiltrates (often viral or atypical)
First Aid cross-reference: Check First Aid sections on Respiratory Infections and Gram-positive/atypical organisms (Pneumococcus, Mycoplasma, Legionella).
Etiology: The CAP bugs you must recognize
Typical vs atypical (board-style mental model)
Typical CAP → productive cough + lobar consolidation
Atypical CAP → “walking pneumonia,” dry cough + diffuse interstitial pattern
High-yield CAP pathogens (with telltale clues)
| Pathogen | Classic clues | Key Step facts |
|---|---|---|
| Streptococcus pneumoniae (alpha-hemolytic, lancet diplococci, optochin sensitive, bile soluble) | Most common CAP overall; sudden fever, productive cough, pleuritic pain | Encapsulated → ↑ risk in asplenia/sickle cell, alcoholism; vaccine prevents |
| Haemophilus influenzae (coccobacillus) | COPD, smokers; can worsen chronic bronchitis | Can be encapsulated (type b) → vaccine; often causes pneumonia + otitis/sinusitis |
| Mycoplasma pneumoniae (no cell wall) | Young adults, dorms/military; gradual onset, dry cough | Cold agglutinins (IgM); atypical; treat with macrolide/doxy (β-lactams don’t work) |
| Chlamydophila pneumoniae | Hoarseness, sore throat, subacute cough | Intracellular-ish; atypical |
| Legionella pneumophila | Hotel/cruise, contaminated water/AC; high fever, GI symptoms, confusion | Hyponatremia, watery diarrhea; silver stain; treat with macrolide/fluoroquinolone |
| Staphylococcus aureus | Post-influenza pneumonia; severe, necrotizing | Think cavitation, abscesses; concern for MRSA depending on scenario |
| Respiratory viruses (influenza, RSV, etc.) | URI prodrome, myalgias | Predispose to secondary bacterial pneumonia (often S. aureus, S. pneumoniae) |
Clinical presentation (how vignettes hand you the diagnosis)
Typical symptoms/signs
- Fever, chills
- Cough (productive more “typical,” dry more “atypical”—but not absolute)
- Dyspnea, tachypnea
- Pleuritic chest pain
- Fatigue, malaise
Physical exam findings you should connect to consolidation
Consolidation conducts sound better:
- Bronchial breath sounds
- Egophony (“E → A”)
- Increased tactile fremitus
- Dullness to percussion
- Crackles (rales)
Board trap: These are also seen with alveolar filling processes generally—pneumonia is the classic cause.
Diagnosis: What to do first + what tests mean
Step-style approach
- Clinical suspicion based on symptoms + exam
- Confirm with chest X-ray (CXR)
- Decide on severity (outpatient vs inpatient/ICU)
- Consider microbiologic tests if severe/complicated
Imaging patterns
- Lobar consolidation: typical bacterial (esp. pneumococcus)
- Patchy multifocal opacities: bronchopneumonia (staph/gram-negatives)
- Interstitial infiltrates: atypical/viral
- Cavitation/abscess: aspiration (anaerobes), S. aureus, Klebsiella (also classically currant jelly sputum)
Labs (high yield)
- CBC: leukocytosis common
- Procalcitonin: tends to rise in bacterial infection (not perfect—don’t over-trust on exams)
- Blood cultures / sputum culture: most useful in severe CAP, ICU, immunocompromised, or when MRSA/Pseudomonas concern
Legionella tell (classic triad-ish)
- Pneumonia + GI symptoms (diarrhea) + hyponatremia ± confusion
If you see this, think Legionella and treat with azithro or levofloxacin.
Treatment: Empiric therapy (Step 1–friendly)
For Step 1, focus less on guideline nuance and more on matching coverage to likely pathogens.
Outpatient CAP (otherwise healthy)
- Macrolide (e.g., azithromycin) or
- Doxycycline
Why: good atypical coverage + reasonable typical coverage.
Outpatient with comorbidities (COPD, diabetes, etc.)
- Amoxicillin/clavulanate + macrolide/doxy
or - Respiratory fluoroquinolone (levofloxacin, moxifloxacin)
Inpatient (non-ICU) CAP
- β-lactam (e.g., ceftriaxone) + macrolide or
- Respiratory fluoroquinolone
ICU/severe CAP (conceptual)
- β-lactam + macrolide (or β-lactam + respiratory FQ)
When to add MRSA or Pseudomonas coverage (Step triggers)
Add coverage if there are risk factors (recent influenza with severe pneumonia for MRSA; structural lung disease for Pseudomonas; recent hospitalization/IV antibiotics; known colonization).
- MRSA coverage: vancomycin or linezolid
- Pseudomonas coverage: antipseudomonal β-lactam (e.g., piperacillin-tazobactam, cefepime, meropenem) + additional agent depending on severity
Pharmacology tie-ins (very testable)
Atypicals = no cell wall / intracellular-ish → β-lactams may fail
- Mycoplasma has no peptidoglycan cell wall → penicillins/cephalosporins ineffective
- Treat atypicals with:
- Macrolides (bind 23S rRNA of 50S; QT prolongation, GI upset)
- Tetracyclines (30S; photosensitivity, teeth/bone effects, contraindicated in pregnancy/children)
- Fluoroquinolones (DNA gyrase/topo II; tendon rupture, QT prolongation; avoid in pregnancy/children)
High-yield associations and “classic Step stems”
Encapsulated organism: S. pneumoniae
Risk groups:
- Asplenia (functional or anatomic): sickle cell disease, splenectomy
- Alcohol use disorder
- Older adults
Mechanism to remember: Capsule resists phagocytosis → need opsonization (IgG, C3b).
Patients with impaired opsonization/clearance get slammed by encapsulated bugs.
Post-influenza pneumonia: S. aureus
Stem: patient had influenza last week → now very sick, high fever, productive cough, hypoxia, possible cavitation.
Think: secondary bacterial pneumonia (often S. aureus, also S. pneumoniae).
Legionella = water exposure + GI + low sodium
Stem: convention/hotel, cruise ship, AC system, hot tubs.
Pearl: Often doesn’t respond to β-lactams; choose macrolide or fluoroquinolone.
Aspiration masquerading as CAP
Stem: alcoholic or seizure → pneumonia in dependent lung segments (e.g., right lower lobe) + foul-smelling sputum.
Bug type: anaerobes; complication: lung abscess.
Complications (know the big ones)
- Parapneumonic effusion / empyema (pus in pleural space)
- Lung abscess (aspiration, S. aureus, Klebsiella)
- Sepsis
- ARDS (severe inflammatory lung injury)
- Respiratory failure in patients with limited reserve (COPD, CHF)
Rapid review table (what to memorize)
| Pattern | Likely cause | What they’ll test |
|---|---|---|
| Lobar consolidation, sudden onset, rusty sputum | S. pneumoniae | Encapsulated, optochin sensitive, vaccine, asplenia risk |
| Walking pneumonia, dry cough, young adult | Mycoplasma | No cell wall, cold agglutinins, macrolide/doxy |
| Severe pneumonia + diarrhea + confusion + hyponatremia | Legionella | Water exposure, silver stain, macrolide/FQ |
| After influenza, necrotizing/cavitary pneumonia | S. aureus | Secondary bacterial pneumonia, MRSA considerations |
| COPD/smoker with pneumonia | H. influenzae | COPD association, vaccine concept (type b) |
First Aid cross-references (where this lives in your book)
Use these sections to anchor your review:
- Microbiology → Gram-positive cocci: Streptococcus pneumoniae, Staphylococcus aureus
- Microbiology → Atypical organisms: Mycoplasma, Chlamydophila, Legionella
- Respiratory system → Pulmonary infections/pneumonia patterns
- Immunology: encapsulated organisms; asplenia; opsonization (IgG, C3b)
(Section titles vary slightly by edition, but these headings are consistent.)
USMLE-style takeaways (the “if you remember nothing else” list)
- Most common CAP: Streptococcus pneumoniae (encapsulated; asplenia risk).
- Atypicals often cause dry cough + interstitial infiltrates; treat with macrolide/doxy/fluoroquinolone, not β-lactams.
- Legionella = water exposure + GI symptoms + hyponatremia → azithro or levofloxacin.
- Post-influenza severe pneumonia → think S. aureus (can be necrotizing).
- Pneumonia causes hypoxemia mainly via V/Q mismatch (shunt physiology).