Plant sugar metabolized by gut bacteria to toxic hydrogen sulfide

The degradation of the dietary sugar sulfoquinovose by anaerobic bacteria to toxic hydrogen sulfide was described for the first time - increased production of hydrogen sulfide in the human intestinal system has been associated with inflammatory bowel disease and colon cancer.

Sulfoquinovose is a sugar found in plants, which contains sulfur. As a constituent of green-vegetable diets, for example in spinach and salad, it is also found in the human intestinal system - an environment without oxygen - and therefore the researchers wanted to answer the following question: What happens when anaerobic bacteria degrade sulfoquinovose in the absence of oxygen? They discovered a new type of metabolism that transforms sulfoquinovose into hydrogen sulfide (H2S). So far, these results have been obtained from a laboratory model system. In future studies, the researchers will have to examine whether sulfoquinovose in the intestine is indeed metabolized to hydrogen sulfide, which is a toxic compound for humans. The results have been published in the journal Frontiers in Microbiology.

This is an entirely novel bacterial degradation pathway that involves three individual discoveries: the discovery of a new link in the biological sulfur cycle, the discovery of a new type of fermentation in Escherichia coli, the best-studied model organism that was also used in this study, and the discovery of a so far unknown energy metabolism in sufite-respiring bacteria, in the Desulfovibrio species.

"Without oxygen, the degradation pathways are completely different. In the context of sulfoquinovose, we discovered a novel type of fermentation in Escherichia coli" says the graduate who condiucted the work. Along with the sulfur-containing degradation product that is formed in this first degradation step, dihydroxypropane sulfonate, the researchers found a second bacterium, Desulfovibrio, which can utilize this intermediate for anaerobic respiration, the so-called sulfite reduction. This type of respiration with the organically-bound sulfur as electron acceptor instead of oxygen is described in detail for the first time. "We thought that hydrogen sulfide may be the end product, but it had never been proven before, and no one knew which bacteria and enzymes may catalyse these reactions", the biologist says.

The next step now is to transfer the results of the laboratory model to the human intestine. "We want to investigate if these degradation pathways can also be found in the intestine and how much they contribute to the overall production of hydrogen sulfide, depending on the diet", says the senior researcher.. Previously it was assumed that organosulfonate substrates, such as taurine, are transformed into hydrogen sulfide mainly from meat-rich and high-fat diets. The new findings now suggest that organosulfonates from vegetarian food, that is sulfoquinovose, can be degraded to hydrogen sulfide as well.

The production of hydrogen sulfide in general can contribute to inflammatory bowel disease and colon cancer. However, it is also assumed that hydrogen sulfide, at least at low concentrations in the intestine, could as well have beneficial effects for our health.