Interactions between leukocytes and anaerobic bacteria in polymicrobial surgical infections

Anaerobic bacteria are frequent isolates from the mixed bacterial flora of surgical infections. Recent studies have defined an important role for these microorganisms in determining the overall virulence of these infections. One mechanism underlying this effect is the ability of anaerobes to interact with leukocytes, resulting in impairment of host defense mechanisms. This review will address two such mechanisms. First, short-chain fatty acids generated by Bacteroides species during the stationary phase of culture have been shown to cause global impairment of the microbicidal activity of neutrophils. The observation that inhibition was maximal at low extracellular pH led to the finding that the fatty acids mediated this effect by shuttling protons from the extracellular to cytoplasmic space, thereby causing intracellular acidification with resultant cell dysfunction. Second, local fibrin deposition at the site of infection appears to impair bacterial clearance. Interaction between Bacteroides species and peritoneal macrophages has been shown to induce cell-associated procoagulant activity. This may represent another potential mechanism by which anaerobes impair leukocyte function and predispose to abscess formation.