Despite the heterogeneity of anaerobes, there are several properties that characterize anaerobic infections. Unlike many other infectious processes in which a single bacterial species causes one illness, anaerobic infections frequently involve multiple bacterial species. In some infections, aerobic or facultative organisms are present together with anaerobes. Because anaerobes often require specialized antimicrobial chemotherapy, it is important that the microbiology laboratory not overlook them after identification of more easily cultured aerobic species. Conditions that often result from polymicrobial or synergistic infection include aspiration pneumonias, lung abscesses, empyemas, intra-abdominal abscesses, pelvic infections, and certain necrotizing infections of skin and subcutaneous tissues.
Many anaerobic infections originate near a disrupted mucosal surface.
Because anaerobes predominate in the indigenous microbiota of the oropharynx, colon, and female genital tract, most anaerobic infections are endogenous in origin. Thus, these infections depend on a primary insult to the patient's defense mechanisms rather than on transmission of a virulent agent from the outside environment. Anaerobic processes often begin as closed-space infections but frequently produce progressive necrosis and burrow through tissue planes. The presence of gas in tissues should raise the question of anaerobic or synergistic infection because many anaerobes are gas producers. Septic thrombophlebitis is a feature of anaerobic infections, especially those infections caused by B. fragilis .