Therapy of mixed anaerobic-aerobic infections. Lessons from studies of intra-abdominal sepsis.

Mixed anaerobic-aerobic infections may occur in a variety of anatomical locations and are usually related to the spread of bacteria from a proximal mucosal surface. Much has been learned about the pathophysiology and treatment of mixed anaerobic-aerobic infections from the study of intra-abdominal sepsis related to spillage of colonic contents. There is an average of five microorganisms at the infected site: three anaerobic and two aerobic pathogens. Appendicitis and diverticulosis are the most common conditions which predispose to fecal contamination of the intra-abdominal cavity. Initially, peritonitis develops which, if untreated, progresses to an abscess. The abscess is a unique pathologic entity which may form a protective environment for the microorganisms and present a barrier to the action of certain antimicrobial agents. Treatment usually involves two modalities: surgical drainage or debridement and appropriate antimicrobial agents to cover both components of the mixed infection. On the basis of in vitro susceptibility there are six groups of antimicrobial agents that are useful in treating intra-abdominal infections: clindamycin; metronidazole; chloramphenicol; broad-spectrum penicillins (carbenicillin/ticarcillin/piperacillin); cephalosporins (cefoxitin/moxalactam); and imipenem. Randomized prospective studies have shown that the first five of these groups are effective in the therapy of intra-abdominal mixed infections. Preliminary data indicate that imipenem/cilastatin is very effective in the therapy of this serious infection; however its place in the therapeutic armamentarium awaits the completion of randomized prospective studies against established drugs.