Significance of Bacterial Flora in Abdominal Irradiation-induced Inhibition of Lung Métastases

We have previously reported that abdominal irradiation prior to i.v. injection of syngeneic tumor cells reduced métastases in lung. Our report described an investigation of the significance of intestinal organisms in the radiation effect. We found that eliminating intestinal organisms with antibiotics totally abolished the radiation effect. Monoassociation of germ-free mice revealed that the radiation effect was observable only for Enterobacter cloacae, never for Streptococcus faecium, Bifidobacterium adlesentis, or Escherichia coli. After abdominal irradiation of regular mice, E. cloacae multiplied in cecal contents, adhered to mucous mem branes, invaded the cecal wall, and translocated to mesenteriilymph nodes. Intravenous administration of E. cloacae in place of abdominal irradiation inhibited métastases. /-.'.c/oocoe-monoassociated mice devel oped fewer métastases than germ-free mice, and the reductionwas further enhanced by abdominal irradiation. We concluded that abdominal irra diation caused the invasion of E. cloacae from the mucous membrane of the intestine and inhibited formation of lung métastases.