Starch-degrading gut microbes Ruminococcus bromii and Bifidobacterium adolescentis differ in their ability to degrade resistant starch type 3.

Intrinsic resistant starch type 3 (RS-3) is retrograded starch that is highly resistant to pancreatic digestion (≥80% RS) and will therefore transit to the colon largely intact. Two gut microbes, known as RS degraders, Ruminococcus bromii ATCC27255 and Bifidobacterium adolescentis L2-32, were studied for their ability to degrade intrinsic RS-3 with defined crystal type and chain length (A-type, degree of polymerisation (DP) 16 or DP 21; B-type, DP 32 or DP 76). Remaining glucose, malto-oligosaccharides and non-degraded insoluble RS-3 were quantified over time and remaining RS-3 was visualized by Scanning Electron Microscopy (SEM) over time and compared to degradation of granular maize and potato starch. R. bromii was not limited by any specific physico-chemical starch characteristic and degraded all substrates gradually to primarily maltose and glucose, although these sugars were not further utilised. In contrast, B. adolescentis was unable to degrade B-type intrinsic RS-3 and only slightly fermented A-type intrinsic RS-3 to acetate, whereas granular maize and potato starch were fermented readily to acetate and lactate. The extensive use of SEM in this study revealed the unique morphology of the RS-3 structures and the difference in degradation approach by the two gut microbes. It can be concluded that efficient degradation of intrinsic RS-3 requires microbes with specific enzyme machineries such as those present in R. bromii.