Effect of gentamicin and dexamethasone on the natural history of the rat Escherichia coli brain abscess model with histopathological correlation.

In a rodent model of Escherichia coli brain abscess, the natural history of the infection was studied and the influence of a glucocorticoid (dexamethasone) and an antibiotic (gentamicin) on the development of brain abscess and the survival of abscess-bearing animals was evaluated. The administration of steroids using three different dosage schedules suppressed the macrophage and glial response, decreased collagen formation, increased the number of pathologically evident bacteria, and decreased host survival. The administration of antibiotics by parenteral routes decreased the number of viable bacteria in the abscess. The simultaneous administration of systemic gentamicin and dexamethasone resulted in increased host survival to a level that was intermediate between that of animals treated with dexamethasone alone and that of those treated with gentamicin alone. Thus, some of the adverse effect of corticosteroids on host survival could be mitigated by the simultaneous administration of antibiotics. Finally, it was observed that the abscess in this model tends to expand along white matter tracks. This path of least resistance may be responsible for the observation that brain abscesses tend to rupture into the ventricle rather than into the subarachnoid space via the cortex.