Effect of antimicrobial therapy on bowel flora and bacterial infection in irradiated mice.

Mice exposed to 10 Gy cobalt-60 radiation were given intramuscular antimicrobial therapy of gentamicin, or metronidazole, or a combination of the two. Mortality in the mice treated with metronidazole alone or in combination with gentamicin occurred earlier than in the controls (P less than 0.001). Microorganisms were recovered from the blood, spleen, and liver of the metronidazole-treated mice earlier than from other groups. The predominant organisms recovered from these animals were Enterobacteriaceae. Quantitative cultures of the ileal flora showed a decrease in the number of aerobic, facultative anaerobic and strict anaerobic bacteria after irradiation, and a subsequent increase only in the number of strict aerobic bacteria. As compared to untreated mice, a rapid decrease (by 8.8 logs) in the number of anaerobic flora occurred in the mice treated with metronidazole 5 days after irradiation. This was followed by a rapid increase in the number of aerobic organisms which coincided with the earlier mortality in this group. These data suggest that antimicrobial agents that decrease the number of the strict anaerobic component of the gut flora enhance systemic infection by aerobic or facultative anaerobic bacteria, and this facilitates mortality after irradiation.