Spontaneous polyposis in the small intestine of germ-free and conventionalized BALB/c mice.

The spontaneous polyposis in the small intestine of germfree (Gf) and conventionalized (Cv) BALB/c mice was studied. Gf mice were bred in our laboratory and maintained Gf in vinyl isolators. The first generation offspring of the Cv mice derived from the Gf mice was used as Cv animals. When they were 12 months old, the animals were killed under CO2 inhalation and autopsied carefully for the number and size of polyps with the aid of a dissecting microscope. The incidence of polyposis was higher in the Gf mice (68% in female and 89% in male) than in the Cv mice (37% in female and 51% in male). The number of polyps/mouse was also higher in the Gf mice (2.3 in female and 5.7 in male) than in the Cv mice (0.8 in female and 1.3 in male). All of the polyps were histopathologically adenomatous and developed only in the upper part (mainly duodenum) of the small intestine. The present study demonstrated that development of polyposis in the small intestine of BALB/c mice was suppressed by the presence of intestinal microflora.