Comparative efficacies of ticarcillin, ticarcillin/clavulanic acid, piperacillin and cefoxitin against polymicrobial infections in mice caused by escherichia coli and bacteroides fragilis

A model of localised abscess formation was used to establish mixed infections caused byEscherichia coli andBacteroides fragilis. The β-lactamase producing, ticarcillin-resistant strainsE. coli E96 andB. fragilis VPI 8708 were used to produce one infection, and in another infection, a β-lactamase hyperproducing strainE. coli 41548 was combined with a ticarcillin-susceptible strain,B. fragilis B3. Treatment, at doses producing clinically achievable concentrations in mouse serum, began 1 h after inoculation, and continued three times daily for four days. Bacterial numbers in infected tissue were measured at intervals. Against both infections, ticarcillin was ineffective in preventing bacterial growth and abscess formation in all mice. Piperacillin prevented abscess formation in 60% of the mice infected withE. coli E96/B. fragilis VPI 8708, and in 40% of those in theE. coli 41548/B. fragilis B3 group. Therapy with ticarcillin/clavulanic acid or cefoxitin reduced the number of both organisms at the site of infection, and thus prevented abscess formation in 100% treated animals.