It is well known that a fibre-rich diet has beneficial effects on human health. What is less well known is that these effects are co-mediated by the gut microbiota, which converts dietary fibre into metabolites with unique physiological functions. And very few know how exactly gut microorganisms can utilize these complex carbohydrates. The molecular underpinnings of this process take us back to the beginnings of microbiome research and the laboratory of avant-garde researcher Abigail Salyers in Urbana, Illinois, United States in the late 1980s.
At that time, microbiology was dominated by the simplistic notion that a few model bacteria could help us understand the entire bacterial diversity. Abigail Salyers and her laboratory, instead, focused on anaerobic bacteria of the genus Bacteroides from the human colon. Her laboratory was particularly interested in how Bacteroides thetaiotaomicron degrades starch and other complex polysaccharides, a highly unusual research topic in those days. Using biochemical assays, they had already convincingly demonstrated that starch-degrading enzymes are associated with cells rather than being extracellularly secreted. In addition, they had shown that starch utilization included a binding step to a protein at the cell surface. They assumed that polysaccharides are subsequently transported across the outer membrane into the periplasmic space, where they are then degraded. Such an import process would ensure that only bacteria catabolizing starch profit from the degradation products, thus avoiding cross-feeding of other bacterial species of the microbiota and providing starch-utilizing bacteria with the ability to occupy a unique metabolic niche in the human gut. However, evidence for binding of starch to a cell surface site does not prove that such a binding is necessary for starch utilization. For the ultimate proof, it was important to show in intact cells that starch can only be used after binding. To address this, the genetic tools for Bacteroides, in particular transposon mutagenesis with Tn4351, which the laboratory had developed in previous years, proved to be groundbreaking. By characterizing a set of B. thetaiotaomicron transposon mutants, which could not grow on starch, Salyers and her team were finally able to elegantly show that binding of starch to the cell surface is required for starch utilization, and that the genes encoding binding site components are under the same regulatory control as the degradative enzymes. Although they were unable to grow on starch, the receptor mutants were still able to grow on shorter-chain oligosaccharides. This was to be expected, as the biochemical analyses had already revealed that the receptor prefers longer-chain polysaccharides.
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