The role of Candida albicans in the pathogenesis of experimental fungal/bacterial peritonitis and abscess formation.

The recovery of Candida albicans along with bacteria from the abdomen in the setting of peritonitis is becoming increasingly common. It is not known whether the interactions between the fungal and bacterial elements of these infections are synergistic, competitive, or neutral. To study this question, we have examined the effects of both the addition of C. albicans to a solely bacterial infection caused by Escherichia coli and Bacteroides fragilis, and the deletion of various components of this system using directed antimicrobial therapy. In a mixed infection, both C. albicans and bacteria contributed to mortality, since only the combination of cefoxitin and amphotericin B improved survival (from 50% to 90%). The addition of C. albicans to the bacterial inoculum increased the recovery of abscesses, but only to the number seen with fungal infection alone, implying two fairly independent processes. Although the number of bacteria recovered from abscesses at 10 days postinfection was unchanged with the addition of fungi, the deletion of the bacterial component of mixed infections led to the overgrowth of C. albicans. We conclude that this model of mixed C. albicans/E. coli/B. fragilis peritonitis is best characterized as two nonsynergistic, parallel infections with incomplete competition, allowing the survival of all three organisms to eventual abscess formation.