A 72-year-old woman with COPD and recent hospitalization (discharged 3 days ago) presents with fever, purulent sputum, and new infiltrates on chest X-ray. Which is the most common causative organism in hospital-acquired pneumonia (HAP) in this patient?

Explanation

## Most Common Cause of HAP **Key Point:** *Staphylococcus aureus* (including MRSA) is the most common causative organism in hospital-acquired pneumonia (HAP) overall, particularly in patients with recent hospitalization, COPD, and healthcare exposure, according to current ATS/IDSA HAP guidelines (2016) and Harrison's Principles of Internal Medicine. ### Epidemiology of HAP Pathogens | Organism | Frequency in HAP | Risk Factors | Resistance Profile | |----------|------------------|--------------|-------------------| | **Staphylococcus aureus (MRSA)** | Most common overall | Recent hospitalization, COPD, prior antibiotics, ICU | β-lactam resistant (MRSA) | | Pseudomonas aeruginosa | Common (especially VAP/late-onset) | Mechanical ventilation, prolonged ICU stay, structural lung disease | Multi-drug resistant (MDR) | | Klebsiella pneumoniae | 10–20% | Aspiration risk, immunocompromised | Variable resistance | | Acinetobacter baumannii | 5–15% | Prolonged ICU stay, mechanical ventilation | Highly resistant (MDR/XDR) | | Enterobacteriaceae (E. coli) | 10–20% | Aspiration, immunocompromised | Variable resistance | **High-Yield:** Per the 2016 ATS/IDSA HAP/VAP guidelines and Harrison's 21e, *Staphylococcus aureus* (including MRSA) is the **most common** pathogen isolated in HAP across multiple large surveillance studies. *Pseudomonas aeruginosa* is the most common gram-negative rod and predominates in VAP and late-onset HAP (>5 days in ICU), but *S. aureus* leads overall HAP pathogen frequency. ### Clinical Context for This Patient - **Recent hospitalization** (discharged 3 days ago): qualifies as healthcare-associated exposure, placing her at risk for MRSA and MDR organisms - **COPD**: structural lung disease increases susceptibility to *S. aureus* and gram-negatives - **Fever + purulent sputum + new infiltrates**: classic HAP presentation - **3-day post-discharge timing**: early HAP window where *S. aureus* (including MRSA) is the dominant pathogen ### Why Other Organisms Are Less Likely as the Single "Most Common" - *Pseudomonas aeruginosa*: Most common gram-negative in HAP/VAP, but overall (gram-positive + gram-negative combined), *S. aureus* leads; *Pseudomonas* dominates in late-onset HAP (>5 days) and ventilator-associated pneumonia. - *Klebsiella pneumoniae*: Important in aspiration and immunocompromised hosts but ranks below *S. aureus* and *Pseudomonas* in overall HAP frequency. - *Acinetobacter baumannii*: Increasingly important in ICU/MDR settings but still less common than *S. aureus* in overall HAP cohorts. ### Mnemonic for HAP Pathogens **"SPACE"** (*S. aureus* is #1 overall): - **S**taphylococcus aureus (MRSA) ← **Most common overall** - **P**seudomonas aeruginosa ← Most common gram-negative / VAP - **A**cinetobacter baumannii - **C**. difficile (toxin-mediated, not typical pneumonia) - **E**nterobacteriaceae (Klebsiella, E. coli) **Clinical Pearl:** The 2016 ATS/IDSA guidelines recommend empiric MRSA coverage (vancomycin or linezolid) for HAP in patients with risk factors including prior hospitalization, COPD, and prior antibiotic use — reflecting the primacy of *S. aureus* in this setting. [cite:Harrison 21e Ch 297; ATS/IDSA HAP/VAP Guidelines 2016, Am J Respir Crit Care Med 194:479–486]