What is the most common cause of empyema?

Commonest Cause of Empyema

Streptococcus pneumoniae is currently the predominant pathogen causing empyema, particularly in children, although this is not always reflected in culture results as many specimens are culture negative due to prior antibiotic use. 1

Microbial Etiology of Empyema

Current Predominant Pathogens

• Streptococcus pneumoniae: Has emerged as the predominant pathogen in empyema, particularly in children 1, 2

  • Found in 75% of culture-negative pleural fluid samples using PCR methods 1
  • Capsular serotype 1 is common (53% of cases) and typically penicillin sensitive 1
  • The widespread use of seven-valent conjugate vaccine has led to emergence of pneumococcal serotypes 1,3, and 19A with higher propensity to cause empyema 3

• Staphylococcus aureus: Second most common pathogen 1, 4

  • More common in:
    • Infants under 6 months of age 1
    • Developing countries, especially during hot/humid months 1
    • Post-traumatic and post-surgical cases 5, 4
  • Includes methicillin-resistant strains (MRSA) 1

Other Important Pathogens

• Streptococcus pyogenes (Group A Streptococcus) 1
• Streptococcus milleri group: Increasing in adults 3
• Anaerobes (Bacteroides, Peptostreptococcus): Associated with aspiration pneumonia 1, 4
• Gram-negative organisms: More common in:
  • Post-surgical cases 5
  • Developing countries 1
  • Examples include Klebsiella, Pseudomonas aeruginosa, Enterobacter, Proteus species 1

Historical Trends in Empyema Pathogens

The microbial etiology of empyema has evolved significantly over time:

• Pre-antibiotic era: S. pneumoniae was the major pathogen, followed by β-hemolytic streptococci and S. aureus 1

• After introduction of sulfonamides and penicillin: Relative proportion of S. aureus increased, especially in the 1950s with rising penicillin resistance 1

• After introduction of penicillinase-stable penicillins: S. pneumoniae has re-emerged as the predominant pathogen 1

• Recent decades: Overall incidence of empyema has been increasing in both children and adults worldwide 3

Diagnostic Challenges

Culture results often underestimate the role of S. pneumoniae because:

• Many patients receive antibiotics before pleural fluid sampling 1, 5
• In a UK multicenter study, only 17% of cases were culture positive 1
• Molecular techniques (PCR) can detect pathogens in approximately 75% of culture-negative cases 1

Clinical Implications

Understanding that S. pneumoniae is the most common cause of empyema has important treatment implications:

• Empiric antibiotic coverage should always include activity against S. pneumoniae 1, 2
• For culture-negative cases, treatment should follow recommendations for community-acquired pneumonia 1
• When S. aureus is suspected (especially in infants, post-surgical cases, or in developing countries), appropriate anti-staphylococcal coverage should be added 1, 2

Conclusion

While the microbiology of empyema varies by age, geographic location, and predisposing factors, S. pneumoniae has re-emerged as the predominant pathogen in most settings, particularly in children with community-acquired empyema. However, clinicians should remain vigilant about other pathogens based on clinical context and local epidemiology.