Organisms reach the brain by one of three known routes: hematogenously (one-third of cases); from contiguous infections of middle ear, sinus, or teeth (one-third of cases); or by direct implantation by neurosurgery or penetrating trauma (about 10 percent of cases). The route is unknown in about 20 percent. Circumstances that reduce oxygenation of brain parenchyma are important predisposing factors for bacterial invasion. For example, spread from a contiguous infection usually involves intervening cerebral thrombophlebitis, with congestive ischemic hypoxemia of tissue destined to become infected. Hematogenous seeding is facilitated by systemic hypoxemia, as in congenital heart diseases with right-to-left shunt and chronic pulmonary suppuration. This is demonstrated by the prominent role of anaerobic bacteria in brain abscesses. The source of brain abscess should be identified for the dual purpose of eliminating the source itself and gaining insight into the probable bacteriologic characteristics of the abscess. For example, gram-negative rods, especially Bacteroides, are the usual pathogens in otogenic brain abscesses, which are typically single and located in the adjacent temporal lobe or cerebellum. Anaerobic and microaerophilic streptococci are the most common pathogens in sinogenic and odontogenic abscesses and are more typically located in the frontal lobes. Abscesses formed from hematogenous spread are often multiple and polymicrobial, with anaerobic and microaerophilic streptococci commonly represented. Staphylococci are typical pathogens in abscesses due to direct implantation. Gram-negative rods are also suspected in cases related to a neurosurgical procedure.

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