PATHOGENS

• In 80-90% of brain abscesses, multiple organisms are recovered by culture (even more with molecular techniques).
• Bacterial:
  • Streptococci are the most common single organisms identified (30-50%), but anaerobic or other aerobic organisms can predominate.
  • Gram negatives are more common in infants.
  • Early infection = cerebritis, subsequent necrosis and capsule formation → abscess.
    • Causes of pyogenic abscess:
      • ~25% unknown source (cryptogenic)
      • ~50% related to the contiguous spread of infection
      • ~25% hematogenous
• Fungal causes include Candida spp., Aspergillus, and Zygomycetes.
• Parasitic causes include protozoa and helminths.
• Source or host specifics:
  • Paranasal sinusitis: microaerophilic (S. intermedius group) and anaerobic Streptpcoccusspp., Haemophilus species, Bacteroides spp, Fusobacterium spp, Prevotella spp.
  • Otogenic infection: aerobic and anaerobic streptococci, Enterobacteriaceae, Pseudomonas aeruginosa, Prevotella spp, B. fragilis.
  • Odontogenic infection: S. viridans and anaerobic streptococci, Bacteroides spp, Fusobacterium spp, Prevotella spp, Actinomyces spp.
  • Endocarditis: Staphylococcus aureus, S. viridans, Enterococcus.
  • Lung abscess: microaerophilic and anaerobic streptococci, Actinomyces species, Fusobacterium species, Nocardia species, Prevotella.
  • Penetrating trauma: Staphylococcus aureus, aerobic streptococci, Clostridium species, Enterobacteriaceae.
  • Postoperative: Staphylococcus epidermidis, S. aureus, Enterobacteriaceae, Pseudomonas aeruginosa.
  • Right to left shunt (congenital heart disease): microaerophilic and aerobic streptococci.
  • Compromised host (AIDS, cancer chemotherapy, chronic steroids, lymphoma): toxoplasmosis, Nocardia, EBV lymphoma, TB, fungal (Aspergillus or other).
    • Solid organ transplant patients: fungal causes = up to 90% of brain abscesses.
  • Immigrant: cysticercosis, echinococcus, TB (tuberculoma).