The intestinal mucosa has a unique immune system composed of a variety of unique immune cell populations which actively react with potentially pathogenic microbes. However, the intestinal immune system need to simultaneously remain tolerant towards the vast majority of non-pathogenic microbiota. Although it still remains unclear how this delicate balance is maintained in the intestine, a combination of gnotobiotic techniques and the latest immunological evidence has contributed to significant advances in our understanding of how host-microbiota interactions modulate mucosal immune systems. In particular, segmented filamentous bacteria and bacterial species falling within clusters VI and XIVa of the class Clostridia have been shown to profoundly affect the generation and function of mucosal immune cells, including Th17 cells and Treg cells.

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The effect of a lactic acid bacterium on diseases caused by immune disorders was evaluated in this study with the aim of developing practical uses for it as a food product. A potential probiotic strain, Lactobacillus gasseri OLL2809, which exhibits a strong immunomodulatory effect, was selected using an allergic mouse model. The oral administration of this strain reduced antigen-specific immunoglobulin E levels and cedar pollen-induced eosinophilia in mice. In addition, a clinical trial revealed reduction of nasal symptoms in patients with Japanese cedar pollinosis. Furthermore, the effect of this strain on endometriosis, which is associated with aberrant immune states in the peritoneal cavity, was assessed. The results show that oral administration of the strain reduced development of endometriotic regions in the endometriosis mouse model. Moreover, a clinical trial involving patients with endometriosis revealed that L. gasseri OLL2809 was effective against endometriosis, especially against menstrual pain and dysmenorrhea. These results suggest that a potential probiotic, L. gasseri OLL2809, alleviates various immune-related diseases by modulating the immune system, facilitating improvements in our quality of life.

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Commensal bacteria in the gut, which is the largest host immune tissue, are critical for the normal function of the mammalian immune system. The Bacteroides genus is one of the most predominant genera of bacteria in the human or murine intestine, with the characteristics and physiological functions of this genus having undergone much investigation in recent years. Intestinal bacteria affect the environment of their host intestine by utilizing the components of food consumed by the host and their metabolites. Bacteroides species metabolize fructooligosaccharides, including their main components, GF2 and GF3. Furthermore, it has been demonstrated that Bacteroides species exhibit immunomodulatory functions. The production of IgA by Peyer’s patch cells induced by Bacteroides species is more effective than that induced by Lactobacillus. In Bacteroides monoassociated mice, Bacteroides species promote IgA production in the large intestine by inducing germinal center formation and increasing the number of IgA⁺ B cells. The bacterial products, e.g., polysaccharide A, derived from Bacteroides species can modulate the T cell-dependent immune reaction for the development and homeostasis of the host immune system, including protection from pro-inflammatory responses. This review provides an overview of growth factors for Bacteroides and immunologic properties, including mechanisms of immune system activation and resulting immunomodulatory effects.

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Patients with Parkinson’s disease (PD) often have constipation and ileus due to autonomic dysfunction with their motor disturbances. We studied the fecal microbiota and fecal characteristics of 8 PD patients (3 men, 5 women, average age of 73.5 years, average duration of illness of 10.5 years, Hoehn and Yahr’s stage of 4 or 5) and 7 normal volunteers (3 men, 4 women, average age of 69.0 years). Although the shape and color of feces of PD patients indicated no specific patterns, the fecal pH of PD patients tended to be alkaline (p<0.01). In total bacterial populations, there was no significant difference between the PD group and the normal group. Veillonella numbers were lower in PD patients, particularly, in patients taking enteral formulas, Veillonella numbers were under the detection limit. The PD group showed a greater number of Clostridium-other than the normal group (p<0.05).

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