<i>Bifidobacterium longum</i> subsp. <i>longum</i> relieves loperamide hydrochloride-induced constipation in mice by enhancing bile acid dissociation

<i>Bifidobacterium</i> species are known for their efficacy in alleviating constipation. This study aimed to compare the constipation-relieving effects of different <i>Bifidobacterium</i> species (<i>Bifidobacterium longum</i> subsp. <i>longum</i>, <i>Bifidobacterium bifidum</i>, <i>Bifidobacterium animalis</i>, <i>Bifidobacterium breve</i>, <i>Bifidobacterium longum</i> subsp. <i>infantis</i>, and <i>Bifidobacterium adolescentis</i>) and to explore the underlying mechanisms from both the bacterial and host perspectives. We evaluated six <i>Bifidobacterium</i> species for their physiological properties, including growth rate, oligosaccharide utilization, osmotic pressure resistance, cell adhesion, and bile acid dissociation capability. Mice with severe constipation induced by loperamide hydrochloride were treated with these bacteria at a density of 10⁹ CFU per mL for 17 days. Gastrointestinal indices such as fecal water content, time to first black stool defecation, and small intestine propulsion rate were measured to assess constipation relief. Microbiome and metabolome (bile acid and tryptophan) analyses were conducted to elucidate the differences in constipation relief among the species. Our results demonstrated that <i>Bifidobacterium longum</i> subsp. <i>longum</i> exhibited superior physiological traits, including rapid growth, extensive oligosaccharide utilization, and high bile salt dissociation capacity. Notably, only <i>Bifidobacterium longum</i> subsp. <i>longum</i> significantly ameliorated constipation symptoms in the mouse model. Furthermore, this strain markedly restored bile acid and short-chain fatty acid levels in the intestines of constipated mice and altered the composition of the intestinal microbiota. These findings suggest that the enhanced efficacy of <i>Bifidobacterium longum</i> subsp. <i>longum</i> in relieving constipation is associated with its ability to modulate intestinal physiology and microbiota structure and metabolism.