The mixed microbial flora responsible for brain abscesses reflects the origin of many of these abscesses from foci of chronic infection in the mastoids, sinuses, or ear. Anaerobic brain abscesses can also be caused by hematogenous dissemination from chronic suppuration in the lung. The role of anaerobes in periodontal disease is gaining increasing attention. Anaerobic bacteria of the upper respiratory tract can invade the lung parenchyma after aspiration. Although these are necrotizing processes, their prognoses have improved greatly as a result of early diagnosis, vigorous antibiotic therapy, and drainage when indicated.
Anaerobes are extremely important in intra-abdominal infections.
The bacterial flora of the stomach and upper small intestine is meager, consisting largely of oral bacteria that have been swallowed. In contrast, the colon is packed with bacteria. Anaerobes often participate in such intra-abdominal infections as diverticular and appendiceal abscesses, peritonitis, and postoperative infections. They are often implicated in liver abscesses but are much less consequential in biliary tract disease. In many intra-abdominal infections, aerobic or facultative organisms such as Escherichia coli , Klebsiella , Proteus , and enterococci are also present.