The administration of a combination of probiotics and prebiotics is expected to be a promising strategy for improving irritable bowel syndrome (IBS) symptoms. This study aimed to investigate the efficacy of a synbiotic containing Lactobacillus paracasei and Opuntia humifusa extract for symptomatic improvement of IBS in a murine model and to evaluate the mechanism underlying the beneficial effects of this synbiotic. A total of 20 male Wistar rats aged 8 weeks with IBS induced by restraint stress were assigned into four groups and administered L. paracasei as a probiotic and O. humifusa extract as a prebiotic for 4 weeks. The primary outcome was stool consistency at week 4. To evaluate the mechanism underlying the beneficial effects of the synbiotic, fecal microbial analysis was conducted, and the serum corticosterone levels, tumor necrosis factor-α (TNF-α) levels in the colon tissue, and expression of tight junction proteins were investigated. All three treatment groups showed significantly lower scores for stool consistency than the control group at week 4 (all p < 0.001). When compared with the control group, the synbiotic groups showed a significantly greater abundance of L. paracasei in fecal microbial analysis, lower serum corticosterone levels, lower TNF-α levels in the colon tissue, and higher expression of tight junction proteins. This novel synbiotic containing L. paracasei and O. humifusa extract can improve the stool consistency in a murine model of IBS. It may be a promising treatment option for IBS, and human studies are warranted.

Non-viable bacteria, referred to as “paraprobiotics,” have attracted attention as potentially safer alternatives to probiotics. The aim of this study was to investigate the efficacy of heat-killed Lactobacillus casei DKGF7 on the symptomatic improvement of irritable bowel syndrome (IBS) in a rat disease model and to elucidate the underlying mechanisms that contribute to the beneficial effects of heat-killed probiotics. Seven male Wistar rats were induced with IBS by restraint stress and administered heat-killed L. casei DKGF7 for four weeks and then compared with seven rats in the control group. Stool consistency measured four weeks after initial treatment was the primary outcome measure. To investigate the mechanism of action of the heat-killed bacteria on IBS, we measured serum corticosterone levels, inflammatory cytokines in colon tissue, and expression of tight junction proteins (TJPs) in the epithelium. The treatment group showed significantly better stool consistency scores than the control group at week 4, as well as at every measured time point (all p values < 0.05). The treatment group showed lower serum corticosterone levels, lower colonic inflammatory cytokine levels, and higher expression of TJPs compared with the control group. Paraprobiotics such as heat-killed L. casei DKGF7 can improve stool consistency in a rat IBS model, which may indicate a potential therapeutic strategy for IBS treatment.