The pre-eminent role of anaerobes in mixed infections

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Abstract
Anaerobic infections are characterized by a conglomerate of micro-organisms, ranging in oxygen sensitivity from aerobic to facultative to obligately anaerobic. The association of aerobes and anaerobes is explained by the high prevalence of anaerobes in the normal flora of the host, from whence these infections arise. Recent investigations have provided insights to the mechanisms that promote the establishment of mixed infections. Individual microbes can contribute virulence factors that permit infection by the other components; the polysaccharide capsule of Bacteroides fragilis serves as an example of a factor that promotes abscess formation. In other situations members of the mixed flora act in a synergistic manner by providing important growth factors to other constituents. A diphtheroid contributes vitamin K to Bact. melaninogenicus in an oral infection. Certain infectious processes are characterized by a ‘cascade effect’ whereby the infection occurs in a series of stages, each being fostered by specific organisms in the flora. In the case of abdominal sepsis, coliforms promote the peritonitis and septic shock phase, which is followed by intra-abdominal abscess related to anaerobes. The anaerobic abscess itself is a protective environment that fosters a mixed infection. It contains large concentrations of organisms that produce β-lactamases and other enzymes that inactivate antibiotics. The thick wall and poor blood supply of an abscess result in decreased penetration of drugs into the structure. Low oxygen tension in these abscesses decreases phagocytosis and intracellular killing. With regard to immunological defences, anaerobic organisms themselves are somewhat resistant to phagocytosis; they also consume opsonins, thereby interfering with phagocytosis of accompanying aerobic bacteria. Several studies, in both animals and humans, suggest that specific suppression of the anaerobic flora by drugs such as the nitroimidazoles and clindamycin can successfully treat a mixed infection. These findings support the hypothesis that anaerobes may be the critical elements in mixed infections.

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