The findings of intestinal microbiota studies conducted in other countries may not be directly applicable to the Japanese, because the intestinal microbiota of the Japanese has a unique composition that differs from that of other countries. Therefore, studies of the intestinal microbiota in the Japanese population are needed. Many factors influence the intestinal microbiota, including lifestyle, body characteristics and condition, and these influences should be considered in studies of intestinal microbiota. This review introduces one of the most important factors affecting the intestinal microbiota: sex differences. Sex differences in the intestinal microbiota of Japanese people are characterized by age; they are largest in the 30s, and then tend to become smaller with increasing age. The existence of sex differences in the intestinal microbiota suggests that sex is a crucial consideration for studies of intestinal microbiota and disease association, prevention, and treatment approaches that target them.

The gut microbiome plays important roles in digestion, metabolism, and immunity, and its association with systemic and metabolic diseases has been highlighted. In recent years, studies of intestinal microbiota using metagenomic shotgun sequencing have progressed, and our laboratory has constructed the database of gut prokaryotic and viral genomes from the gut metagenome of the Japanese population. We have identified population-specific enrichments such as Bacillus subtilis derived from natto, and β-porphyranase, an enzyme involved in the metabolism of laver. Novel viral species were identified in the gut viral genomes, and the association between crAss-like phages and several diseases were also demonstrated. Metagenome-wide association studies of rheumatoid arthritis and systemic lupus erythematosus have also shown species-level associations between the gut metagenome and the disease, and its interaction with the human genome.

We presented the results of an analysis on frailty status and its risk factors among elderly residents participating in the Kyotango longevity cohort. Compared to the non-frail group, the frail group showed significantly lower intake of nutrients such as plant-based protein, minerals including potassium (K) and magnesium (Mg), B vitamins, and dietary fiber. Among food groups, soybeans and soybean products were identified as being significantly less consumed in the frail group. Cluster analysis examining the correlation between diet, nutrient intake, and gut microbiota identified key bacterial genera, including Eubacterium eligens, Christensenellaceae R-7, and UCG-002. The study highlighted the potential of detailed analyses of the relationships between diet, nutrient intake, and the gut microbiota in contributing to frailty prevention, incorporating the latest research findings.

In this study, we isolated strains of Enterococcus faecalis from a total of 113 feces from 11 wild bird species in order to comprehend the distribution of antimicrobial resistance in the natural environment of Japan, and to survey the actual roles of wild birds as reservoirs of resistant bacteria. We also examined the rate of isolation in each bird species, and performed microbial sensitivity tests using the Kirby-Bauer method. E. faecalis strains were isolated from a total of 36 feces from 6 bird species: the Oriental great reed warbler (Acrocephalus orientalis), Japanese bush warbler (Horornis diphone), bull-headed shrike (Lanius bucephalus), Siberian rubythroat (Calliope calliope), black-faced bunting (Emberiza spodocephala) and reed bunting (Emberiza schoeniclus). However, E. faecalis strains were not isolated from 5 bird species: the Oriental greenfinch (Chloris sinica), tree sparrow (Passer montanus), great tit (Parus minor), rustic bunting (Emberiza rustica) and cinnamon sparrow (Passer rutilans). In microbial sensitivity tests using three strains from each bird species, a total of 74 strains were examined. Moderate resistances to erythromycin, teicoplanin and vancomycin were found, and the rates of isolation of resistant strains were 24.3%, 12.2% and 8.1%, respectively. There were no resistant strains in reed bunting and Siberian rubythroat. Some strains were found to have multi-resistance to multiple drugs. These results reflect the strict controls of sewage and livestock excrement by legislation in Japan, and the policy of reviewing the usage of antimicrobial drugs. Surveys of wild birds should be regularly undertaken, including species of wild birds not only in this study, and with a larger survey area.