A pyogenic brain abscess is a localized focus of parenchymal suppuration, usually of bacterial origin. Brain abscess can be bacterial, fungal, or parasitic. In a large number of bacterial abscesses, more than one organism is identified. A variety of bacterial organisms responsible for brain abscesses. Gram negatives are more common in infants.
Otogenic abscesses are usually mixed anaerobic and aerobic.
Sinusitis abscesses contain organisms present in sinus, are usually mixed. Congenital heart disease leads to infections with streptococcus, hemophilus. Trauma leads to S. aureus and epidermidis. The majority of brain abscesses arise by direct spread from paranasal sinuses, middle ear or mastoid infections. Infection spreads by retrograde thrombophlebitis through valveless diploic veins.