Pulmonology
Pneumonia Types
CAP, HAP, aspiration, atypical — different bugs, different drugs
Pneumonia type — where it was acquired — determines the responsible organisms and the empiric antibiotic. CAP is dominated by Strep pneumo and Mycoplasma; HAP by Gram negatives and MRSA; aspiration by anaerobes; atypicals by intracellular pathogens.
- Strep pneumo share of CAP30-50% of identified cases
- HAP definitionOnset ≥ 48h after admission
- Top HAP organismsPseudomonas, Klebsiella, MRSA
- Aspiration floraOropharyngeal anaerobes + streptococci
- Atypical agentsMycoplasma, Chlamydia, Legionella
- Severity scoreCURB-65 / PSI
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Why "type" determines treatment
The lung's microbial enemy depends on where the patient came from. A previously healthy 30-year-old presenting from the community has likely encountered an organism that lives on human mucosa — Streptococcus pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, or a common respiratory virus. A patient hospitalized for five days, whose pharyngeal flora has been replaced by Gram-negative aerobes and resistant Staphylococci over that time, is likely facing one of those nosocomial organisms instead. The same alveolar infection in two settings is a different disease.
Empiric antibiotic choice is therefore made on epidemiological grounds before any culture grows. The cost of guessing wrong is steep — undertreatment risks death within hours in severe pneumonia, overtreatment selects for resistance and risks C. difficile colitis. Modern guidelines explicitly tie antibiotic regimens to the four categories: CAP (severity-graded), HAP/VAP (with antipseudomonal and MRSA coverage where appropriate), aspiration (anaerobe coverage), and atypical (macrolide or fluoroquinolone in combination).
Lobar vs bronchopneumonia vs interstitial
Radiographic pattern hints at organism. Lobar pneumonia — dense consolidation of a single anatomic lobe with air bronchograms — is classically caused by Streptococcus pneumoniae and Klebsiella pneumoniae. The "currant jelly" sputum and bulging fissure of Klebsiella are textbook teaching, even if rarely seen in practice. Bronchopneumonia — patchy peribronchial infiltrates affecting multiple lobes — suggests Staph aureus or Gram-negative organisms, often with abscess formation. Interstitial pneumonia — diffuse reticular infiltrates with relatively preserved alveoli — is the hallmark of viral (influenza, COVID-19) and atypical (Mycoplasma, Pneumocystis) pneumonia. None of these patterns is pathognomonic, but they steer initial empiric choice.
Worked clinical example
A 78-year-old man with mild dementia is admitted from a nursing home with three days of fever, productive cough, and confusion. Two weeks ago he was hospitalized for a UTI. Vitals: T 38.7, HR 108, BP 102/64, RR 28, SpO₂ 87% room air. Examination: right basal crackles, no signs of CHF. WBC 18,500, BUN 9.2 mmol/L, lactate 2.4. Chest x-ray: right lower lobe consolidation. CURB-65: C(1) + U(1) + R(0, RR 28 → no) + B(0) + 65(1) = 3 — severe; ICU admission.
Categorization: this is technically CAP (he was discharged >14 days ago) but he has multiple risk factors for resistant pathogens — recent hospitalization, nursing-home residence, dementia (aspiration risk), elderly. Empiric coverage must therefore extend beyond standard CAP: ceftriaxone + azithromycin would cover typical CAP and atypicals but miss Pseudomonas and MRSA. Choose instead piperacillin-tazobactam (Gram negatives including Pseudomonas, anaerobes for aspiration risk) + vancomycin (MRSA) + azithromycin (atypicals); send sputum culture, blood cultures × 2, urinary Strep pneumo and Legionella antigens. Oseltamivir if influenza testing positive.
Day 2: blood cultures and sputum grow Streptococcus pneumoniae, pan-susceptible. Urinary antigen positive for S. pneumo, negative for Legionella. De-escalate to ceftriaxone monotherapy, with eventual transition to oral amoxicillin once afebrile and tolerating PO. Stop vancomycin and piperacillin-tazobactam. Total course 5-7 days, longer if complicated by empyema. The case illustrates the algorithm — broad empiric coverage, narrow rapidly based on data.
The four major pneumonia types side-by-side
| Type | Acquisition setting | Top pathogens | Typical empiric Rx |
|---|---|---|---|
| CAP (community-acquired) | No recent hospitalization | Strep pneumo (30-50%), Mycoplasma, H. influenzae, viruses | β-lactam + macrolide, or respiratory fluoroquinolone |
| HAP (hospital-acquired) | Onset ≥ 48h after admission | Pseudomonas, Klebsiella, E. coli, MRSA, Acinetobacter | Antipseudomonal β-lactam + vancomycin/linezolid ± aminoglycoside |
| VAP (ventilator-associated) | Onset ≥ 48h after intubation | As HAP, with higher Pseudomonas and Acinetobacter rates | Per local antibiogram; broad until cultures back |
| Aspiration | Impaired airway protection | Anaerobes (Peptostreptococcus, Prevotella, Fusobacterium) + strep | Amoxicillin-clavulanate or ampicillin-sulbactam |
| Atypical | Community (insidious) | Mycoplasma, Chlamydia pneumoniae, Legionella | Macrolide or doxycycline or fluoroquinolone |
| Viral | Community (seasonal) | Influenza, RSV, SARS-CoV-2, parainfluenza | Antivirals (oseltamivir, baloxavir); add antibiotics only for superinfection |
| Opportunistic (immunocompromised) | HIV, transplant, chemotherapy | PJP, CMV, Aspergillus, mycobacteria | Pathogen-specific (TMP-SMX, ganciclovir, voriconazole) |
Why pneumonia type matters
- Mortality. CAP carries 1-5% outpatient mortality but 5-15% inpatient and 30%+ ICU mortality — early correct empiric coverage saves lives.
- Resistance. Inappropriate empiric coverage in nursing-home and post-hospital patients is the single biggest driver of bad outcomes.
- Antimicrobial stewardship. Procalcitonin-guided therapy, biomarker-driven de-escalation, and respiratory pathogen PCR panels shorten courses safely.
- Public health. Legionella outbreaks require institutional reporting and water-system investigation.
- Vaccination. PCV13/PCV20/PPSV23 prevent pneumococcal disease; influenza, RSV, and COVID vaccines prevent severe viral pneumonia.
- Recognition gaps. Aspiration is underrecognized in dementia and stroke patients; recognizing the dependent-segment distribution prevents misdiagnosis.
- Sepsis pathway integration. Pneumonia is the leading cause of sepsis admissions; bundle compliance reduces mortality.
Common misconceptions
- "Cefepime covers everything in HAP." Cefepime misses MRSA and atypicals — vancomycin/linezolid and macrolide/fluoroquinolone fill those gaps.
- "Aspiration pneumonitis needs antibiotics." Pure chemical pneumonitis (witnessed acid aspiration) is often self-resolving; antibiotics are added only when infection supervenes after 48-72 hours or in necrotizing/anaerobic-suggestive contexts.
- "All atypicals are equally treated." Legionella often requires fluoroquinolone or higher-dose macrolide than Mycoplasma; severity-graded.
- "Negative blood and sputum cultures means no pneumonia." Cultures are positive in <30% of CAP; the diagnosis remains clinical-radiographic.
- "Healthcare-associated pneumonia (HCAP) needs HAP-level coverage." HCAP has been retired by guidelines — individual risk assessment for MDR organisms now governs choice.
- "Treat for 14 days minimum." Five days suffices for uncomplicated CAP responding to therapy; longer courses risk resistance and C. difficile.
Frequently asked questions
What separates community-acquired from hospital-acquired pneumonia?
Time of onset relative to hospitalization. CAP develops in a patient who has not been hospitalized within the past 14 days; HAP develops ≥48 hours after admission. The categorical distinction matters because hospital flora — Gram-negative aerobes (Pseudomonas, Klebsiella, Acinetobacter, E. coli) and methicillin-resistant Staphylococcus aureus (MRSA) — colonize the upper airway of admitted patients within days, replacing the more antibiotic-susceptible community flora. Empiric coverage must therefore be broader for HAP/VAP. The older category 'healthcare-associated pneumonia' (HCAP), which captured nursing-home residents and dialysis patients, has been retired by current guidelines in favor of individualized risk assessment for resistant organisms.
Who gets aspiration pneumonia?
Patients with impaired airway protection: altered consciousness (stroke, seizures, alcohol/drug intoxication, post-anesthesia), dysphagia (Parkinson's, ALS, head and neck cancer, dementia), structural problems (esophageal dysmotility, large hiatal hernia, tracheoesophageal fistula), or mechanical airway interruption (NG tubes, endotracheal tubes). Macroaspiration of oropharyngeal contents seeds the lung with anaerobic bacteria from the gum line — Peptostreptococcus, Prevotella, Fusobacterium — along with aerobic streptococci. Dependent lung segments (right lower lobe when supine, posterior segments when prone) develop the infiltrate. Aspiration pneumonitis (the chemical injury of gastric acid) is distinct from aspiration pneumonia (the resulting infection) — antibiotics are needed for the latter, not the former alone.
What makes atypical pneumonia atypical?
Atypical pneumonia is caused by intracellular organisms — Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila — that lack a typical cell wall (Mycoplasma) or replicate inside host cells (Legionella). They produce a clinical syndrome distinct from classical lobar pneumonia: gradual onset, dry cough, headache, myalgia, extrapulmonary features (rash, hepatitis, hyponatremia, diarrhea), and a chest x-ray that looks worse than the patient sounds — patchy or interstitial infiltrates rather than consolidation. Beta-lactams don't cover them because there's no cell wall to target (Mycoplasma) or insufficient intracellular penetration (Legionella). Macrolides (azithromycin), doxycycline, and respiratory fluoroquinolones cover the major atypicals.
When is Legionella suspected?
Severe pneumonia with extrapulmonary features (hyponatremia, diarrhea, confusion, transaminitis, bradycardia despite fever), in a patient with a relevant exposure: hotel stays, cruise ships, hot tubs, cooling towers, hospital water systems. Severity can be marked — multilobar involvement, respiratory failure, ICU admission. Diagnosis: urinary antigen test (rapid, but only detects serogroup 1 — about 80% of cases), or PCR/culture of respiratory specimens. Treatment: levofloxacin or azithromycin; severe disease may warrant combination. Public health notification is required for institutional clusters. Risk factors: smoking, COPD, immunosuppression, older age.
How is severity assessed?
Two validated scores. CURB-65 — one point each for Confusion, Urea >7 mmol/L, Respiratory rate ≥30, Blood pressure (SBP <90 or DBP ≤60), age ≥65 — risk-stratifies for outpatient (0-1), inpatient (2), and ICU (≥3) management. The Pneumonia Severity Index (PSI) is more granular, incorporates comorbidities and labs, and predicts 30-day mortality across five classes. Sepsis criteria (qSOFA, SIRS) overlay these for septic patients. Lactate, procalcitonin, and biomarkers add nuance. The most clinically important real-time question is: does the patient need ICU-level monitoring? Respiratory rate ≥30, septic shock, multilobar disease, severe hypoxia, and acute organ dysfunction all argue yes.
What empiric antibiotics do you give?
Outpatient CAP without comorbidities: amoxicillin (high dose) or doxycycline, or a macrolide where local resistance is <25%. Outpatient CAP with comorbidities: amoxicillin-clavulanate + macrolide/doxycycline, or a respiratory fluoroquinolone (levofloxacin, moxifloxacin) as monotherapy. Inpatient non-ICU CAP: IV β-lactam (ceftriaxone, ampicillin-sulbactam) + macrolide, or a respiratory fluoroquinolone. ICU CAP: β-lactam + macrolide (or fluoroquinolone), with anti-MRSA and antipseudomonal coverage if risk factors present. HAP/VAP: vancomycin or linezolid + antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, meropenem) ± aminoglycoside per local antibiogram. Aspiration: amoxicillin-clavulanate or ampicillin-sulbactam (anaerobic coverage). De-escalate rapidly based on cultures.
Should we still treat viral pneumonia with antibiotics?
Viral pneumonia — influenza, RSV, SARS-CoV-2, adenovirus, parainfluenza — is more common than once recognized; PCR panels now detect it routinely. Pure viral pneumonia does not require antibacterial coverage, but bacterial superinfection (especially S. aureus following influenza) is a recognized complication and is suggested by clinical worsening, new fever, or purulent sputum after an initial improvement. Procalcitonin <0.25 ng/mL supports a viral etiology and a decision to withhold or stop antibiotics. The combination of oseltamivir (within 48 hours for influenza) and antibiotics only if bacterial features develop is the modern standard. Empiric overtreatment with antibiotics drives resistance and C. difficile risk.