Organism

Brain abscess

Often polymicrobial (14–28% of cases).

Child: anaerobes (40%), aerobic and anaerobic viridans streptococci (GPC in chains), S. aureus (GPC), S. pneumoniae (GPDC), S. pyogenes (GPC in chains); less common: Enterobacteriaceae (GNR), P. aeruginosa (GNR), H. influenzae (GNCB), N. meningitidis (GNDC).

Adults: Viridans streptococci (anaerobic Streptococcus anginosus [ milleri] group) (70%).

Enterobacteriaceae (GNR) (23–33%), S. aureus (GPC) (10–15%), N. meningitidis , Listeria sp., anaerobes (20–40%) including bacteroides (GNR), prevotella (GNR), fusobacterium (GNR), eubacterium (GPR), and propionibacterium (GPR). More rarely identified: S. pneumoniae , Rhodococcus sp., Group B streptococci (GPC in chains), nocardia (GPR), actinomyces (GPR), parasites, T. solium (cysticerci), Entamoeba histolytica , Schistosoma sp., and fungi (1%).

Immunocompromised: T. gondii, Cryptococcus neoformans, nocardia (GPR), Listeria sp. (GPR), mycobacteria (AFB), Aspergillus sp., C. albicans , coccidioides, Zygomycetes (mucor, rhizopus), E. histolytica.

Posttraumatic: S. aureus (GPC), viridans streptococci (GPC in chains), Enterobacteriaceae (GNR), coagulase-negative staphylococci (GPC), Propionibacterium acnes (GPR).