The treatment of irradiated mice with polymicrobial infection caused by Bacteroides fragilis and Escherichia coli.

The effects on the faecal flora and the efficacies of various antibiotic regimens administered as treatment for a mixed infection caused by Bacteroides fragilis and Escherichia coli in the irradiated host were investigated in a subcutaneous abscess model with C3H/HeN mice which had been exposed to 60Co. The regimens used included imipenem, ofloxacin, metronidazole and the combination of ofloxacin and metronidazole. The incidence of mortality associated with each regimen was as follows: 2 of 20 (10%) mice treated with ofloxacin; 16 of 20 (80%) given metronidazole (P < 0.05 compared with ofloxacin); 8 of 20 (40%) given imipenem (P < 0.05 compared with ofloxacin); 5 of 20 (25%) given the combination of ofloxacin and metronidazole (P = NS compared with ofloxacin); and 15 of 20 (75%) controls given a placebo (P < 0.05 compared with ofloxacin). Compared with the control group, the mean number of lactose-fermenting facultative anaerobes in the faeces of irradiated mice decreased significantly during treatment with either ofloxacin or ofloxacin combined with metronidazole; rose significantly from day 6 during treatment with metronidazole and was similar during treatment with imipenem. The numbers of strict anaerobes fell with all regimens (including the placebo) but, compared with the control group, were significantly higher in the ofloxacin and imipenem groups and significantly lower in the groups which received metronidazole (with or without ofloxacin). Imipenem and the combination of ofloxacin and metronidazole led to significant reductions in the numbers of cfu of both E. coli and B. fragilis in the abscesses compared with the placebo; ofloxacin alone caused a reduction in the number of cfu of E. coli only and metronidazole alone caused a reduction in the number of cfu of B. fragilis only. While the administration of metronidazole had a favourable effect on the B. fragilis component of the localized infection, there was a higher incidence of mortality with this agent, probably because it led to an increase in the number of Enterobacteriaceae in the faeces at the expense of a reduction in the number of strict anaerobes. Antimicrobial agents such as imipenem or the combination of a quinolone, such as ofloxacin, and metronidazole may be used successfully to treat localized infections in the irradiated host while avoiding potentially deleterious increases in the number of facultative anaerobes in the bowel.