Most Common Pathogens in Community-Acquired Pneumonia (CAP) and Hospital-Acquired Pneumonia (HAP)
Streptococcus pneumoniae is the most common bacterial pathogen in CAP, while gram-negative bacilli and Staphylococcus aureus predominate in HAP. 1, 2
Community-Acquired Pneumonia (CAP) Pathogens
Bacterial Pathogens
- Streptococcus pneumoniae: The most common bacterial pathogen in CAP, accounting for approximately 20-60% of cases where a bacterial pathogen is identified 2, 1
- Haemophilus influenzae: Second most common bacterial pathogen (most strains are nontypeable) 2, 3
- Mycoplasma pneumoniae: Important atypical pathogen, especially in outpatients and younger adults (4-39% of cases) 2, 4
- Chlamydia pneumoniae: Another significant atypical pathogen (0-20% of cases) 2
- Moraxella catarrhalis: Less common but still significant, especially in patients with COPD 3
- Staphylococcus aureus: More common in post-influenza pneumonia and in severe CAP 2
- Legionella species: Important cause of severe CAP (5-10% of cases) 2, 4
- Klebsiella pneumoniae and other gram-negative rods: Less common but significant, especially in elderly and those with comorbidities 2
Viral Pathogens
- Recent studies show viruses account for up to 40% of CAP cases with an identified pathogen 5
- Respiratory syncytial virus (RSV): Most common viral pathogen, especially in children 2
- Influenza virus: Seasonally important and can lead to secondary bacterial pneumonia 2
- Other viruses: Parainfluenza, adenovirus, rhinovirus, coronavirus, and metapneumovirus 2
Important Epidemiological Patterns
- Mixed infections are common, occurring in 8-40% of cases, often with viral-bacterial combinations 2
- In 40-60% of CAP cases, no pathogen is identified despite diagnostic testing 2
- Age affects pathogen distribution:
- Younger patients: Higher prevalence of atypical pathogens and viruses
- Older patients: Higher prevalence of S. pneumoniae and gram-negative bacteria 2
Hospital-Acquired Pneumonia (HAP) Pathogens
Predominant Pathogens
- Methicillin-resistant Staphylococcus aureus (MRSA): Most common pathogen in HAP (24.6% of cases) 6
- Pseudomonas aeruginosa: Second most common (18.8% of cases) 6
- Methicillin-sensitive Staphylococcus aureus (MSSA): Accounts for approximately 13.8% of cases 6
- Klebsiella pneumoniae and other Enterobacteriaceae: Common gram-negative pathogens in HAP 2
- Acinetobacter species: Increasingly important, especially in ICU settings 3
Risk Factors for Specific HAP Pathogens
- P. aeruginosa: Prior structural lung disease, corticosteroid use, prior antibiotic therapy, or septic shock on admission 2
- MRSA: Recent hospitalization, antibiotic exposure, and residence in long-term care facilities 6
- Gram-negative bacteria: Increased risk in patients with comorbidities and recent healthcare exposure 6
Clinical Implications
Diagnostic Considerations
- Blood cultures should be obtained in all hospitalized patients with CAP before antibiotic administration 2
- Sputum Gram stain and culture are recommended for hospitalized patients but are optional for outpatients 2
- Testing for viral pathogens (especially influenza and COVID-19) is recommended when these viruses are circulating in the community 5
Treatment Implications
- Initial empiric therapy for CAP should always include coverage for S. pneumoniae 1
- For hospitalized patients without risk factors for resistant pathogens, β-lactam/macrolide combination (e.g., ceftriaxone with azithromycin) is appropriate 5
- HAP requires broader coverage including MRSA and Pseudomonas aeruginosa due to the higher prevalence of these pathogens 6
Antibiotic Resistance Concerns
- Drug-resistant S. pneumoniae is increasing, with resistance rates to macrolides reaching 75% in some regions 4
- Inappropriate initial antimicrobial treatment is more common in HAP than CAP and is associated with increased mortality 6
Understanding the likely pathogens in CAP and HAP is crucial for selecting appropriate empiric antimicrobial therapy and improving patient outcomes.