Striking changes occurred in mid-small intestinal and cecal flora of rats after exposure to 1400 R of whole-body irradiation or 2000 R of intestinal irradiation delivered at 250 kVp and 30 mA. The changes reached a maximum 3 days after irradiation. A consistent 100- to 10,000-fold increase in coliforms and enterococci occurred at both sites, and similar increases in proteus occurred in most animals. The normally predominant lactobacilli decreased slightly in numbers or remained unchanged. Other organisms participated less constantly in the overgrowth. Massive bacterial invasion of the severely injured small intestinal mucosa did not occur. There was no difference in bacterial invasion of mesenteric lymph nodes, spleen, liver, or heart blood between control and irradiated animals. The bacterial overgrowth in the intestines was effectively prevented by orally administered neomycin-polymyxin B-bacitracin without altering mortality. The findings indicate that massive intestinal flora changes occur in rats with the intestinal-type irradiation syndrome, but these changes are probably not significant in causing the animal's death.

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