The mechanisms by which Bacteroides species contribute to the virulence of polymicrobial intraabdominal infections are poorly defined. One potential mechanism may be their ability to inhibit host defenses, thereby permitting their co-pathogens to survive and exert their intrinsic virulence. The purpose of these studies was to examine the effect of the major Bacteroides byproduct, succinate, on the generation of the respiratory burst by neutrophils. Succinate caused a dose-dependent inhibition of both superoxide and hydrogen peroxide production. This phenomenon was markedly pH-dependent, i.e. inhibition occurred following incubation of cells in succinate at pH 5.5 but not at pH 7.4. It was further hypothesized that succinate effected this inhibition by reducing cytoplasmic pH. Presumably, at low pH, succinate in its protonated form traversed the plasma membrane and dissociated within the cytoplasmic space, thereby reducing pH. The cytoplasmic pH was determined fluorimetrically. Succinate-containing medium at pH 5.5 significantly reduced cytoplasmic pH compared to both control medium pH 5.5 and succinate medium pH 7.4. In further support of the hypothesis, it was shown that uptake of 14C-succinate by neutrophils was 8-fold increased at pH 5.5 compared to pH 7.4. Reduced temperature (4 degrees C) protected cells from the inhibitory effect of succinate at pH 5.5. Measurements of cytoplasmic pH and uptake of 14C-succinate suggested that this protection was achieved by retarding the uptake of succinic acid at low temperature. These studies suggest that fatty acid production by Bacteroides species in the acid milieux of an infection may be a potential mechanism by which Bacteroides species enhance the virulence of mixed infections.