Cecal size of axenic ND-2 mice are 2 to 10 times the size of the conventional counterpart and can cause death by volvulus. Association of axenic mice with the microbial cecal flora of the conventional mouse resulted in a reduction of the ceca to normal limits. A strain of Clostridium difficile was isolated from the cecal contents which when associated with axenic mice resulted in a restoration of the ceca to normal size. Two strains of Bacteroides isolated from the cecal contents produced similar results in combination when associated with axenic mice.