Bifidobacterium animalis ssp. lactis GCL2505 has been shown to proliferate in the human intestine. The intestinal dynamics and physiological effects of GCL2505 as well as the mechanism underlying proliferation in the gut were investigated. GCL2505 showed markedly higher resistance to free bile acids (cholic and deoxycholic acids) than other bifidobacterial species. The intestinal dynamics of GCL2505 and B. longum ssp. longum JCM1217^T was compared. The level of B. animalis ssp. lactis in the GCL2505-administered group was remarkably higher than that of B. longum in the JCM1217^T-administered group. The distribution of B. animalis ssp. lactis through the intestine of the GCL2505-administered group revealed that GCL2505 proliferated in the cecum. The physiological effects of GCL2505 and JCM 1217^T were investigated. The cecal IgA level in the GCL2505-administered group was significantly higher than that in the nontreated control group. In contrast, the JCM 1217^T-administered group did not manifest any change in the cecal IgA level. Mucin excretion in the GCL2505-administered group was significantly higher than that in the JCM 1217^T-administered group. The thickness of the sulfomucin layer of the colon in the GCL2505-administered group tended to be higher than that in the JCM 1217^T-administered group. In a loperamide-induced constipation model, fecal excretion in the GCL2505-administered group was significantly increased compared with that in the loperamide-treated control group. Short-chain fatty acid concentration in the GCL2505-administered group was significantly higher than that in the loperamide-treated control group. These results indicate that the level of proliferation of probiotics in the intestine correlates with the magnitude of host physiological responses, such as IgA production and mucin secretion, which possibly affect gastrointestinal functions such as bowel movement to counteract constipation. GCL2505 exhibits high tolerance to secondary bile acids, which partially explains its higher rate of proliferation in the large intestine.

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