Human gut commensal bacterium Ruminococcus species FMB-CY1 completely degrades the granules of resistant starch.

Resistant starch (RS) in the diet reaches the large intestine and is fermented by the gut microbiota, providing beneficial effects on human health. The human gut bacterium FMB-CY1 was isolated and identified as a new species closest to Ruminococcus bromii. Ruminococcus sp. FMB-CY1 completely degraded RS including commercial RS types 2, 3, and 4, and generated glucose and maltose; however, it did not assimilate glucose. Genome analysis revealed 15 amylolytic enzymes (Amy) present in FMB-CY1. The evolutionary trees revealed that the Amys were well divided each other. All Amys (4, 9, 10, 12, and 16) containing cohesin and/or dockerin and scaffolding proteins known to be involved in constituting the amylosome, were identified. A new species of Ruminococcus, strain FMB-CY1, was considered to have the ability to form amylosomes for the degradation of RS. This new RS-degrading Ruminococcus species provides insights into the mechanism(s) underlying RS degradation in the human gut. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10068-021-01027-2.

Comparative study on four amylosucrases from Bifidobacterium species.

Amylosucrase (ASase) is α-glucan-producing enzyme. Four putative ASase genes (bdas, blas, bpas, and btas) were cloned from Bifidobacterium sp. and expressed in Escherichia coli. All ASases from Bifidobacterium sp. (BAS) displayed typical ASase properties with slightly different characteristics. Among the BASs studied, BdAS and BpAS showed maximal enzyme activities at 35 and 30 °C, respectively, whereas BlAS and BtAS were maximally active at higher temperatures, i.e., 45 and 50 °C, respectively. BpAS exhibited optimum pH under slightly basic conditions (pH 8.0), while BdAS, BlAS, and BtAS preferred weakly acidic conditions (pH 5.0-6.0). All BASs showed higher isomerization activities. Particularly, BlAS produced more trehalulose than turanose. Although polymerization was the highest for BtAS, BtAS synthesized α-1, 4-glucans with a lower degree of polymerization than that of the other BASs. The versatile properties of the BASs described could contribute to the efficient production of highly valuable biomaterials for the agriculture, food, and pharmaceutical industries.