Abstract:
The fermentation of carbohydrates is one of the primary functions of the gut microbiome, which results in the production of short-chain carboxylates (SCCs) and gasses such as hydrogen (H2) and carbon dioxide (CO2). Fermentative H2 production and interspecies H2 transfer predominantly drive colonic H2 metabolism rather than respiration. Accumulating H2 disrupts the gut function, harms humans, and needs to be prevented. However, H2 is an important energy source for gut microbes such as sulfate-reducing bacteria, acetogens, and methanogens. Interspecies H2 transfer is a form of microbial syntrophy that dominates in the gut, but its role in modulating overall metabolism and microbial community dynamics is poorly understood. First investigations showed that Christensenellaceae and the archaeal family Methanobacteriaceae cooccur in humans with a lean body mass index. Christensenella minuta is a highly prevalent, heritable, health-associated bacterium from the human gut that cross-feeds H2 to the methanogen Methanobrevibacter smithii. It was previously found that in continuous co-culture, C. minuta produces less n-butyrate when M. smithii is abundant. For Objective 1, we tested if H2-removal by M. smithii leads to the downregulation of n-butyrate production. Therefore, we developed a bioelectrochemical system (BES) that removes H2 by oxidation at the electrode, mimicking a syntrophic microbial partner that takes up H2. The unique design of the BES brings the microbe at a 1 mm distance close to the platinum-doped carbon electrode and provides a large surface area (~122 cm²) to ensure an efficient H2 removal. Thus, it provides an environment favored by H2-producing, carbohydrate-degrading bacteria. For proof of concept, C. minuta was used as an H2-producing microbe. With H2 removal by the BES electrode, C. minuta shifts its metabolism towards more acetate and less n-butyrate production, analogously to when the methanogen is present. Our findings underscore the importance of thermodynamics and H2 transfer in regulating the metabolic output of the microbiota in the human gut. Furthermore, for Objective 2, we wanted to answer the question: Can the H2 removal by the BES compete with a syntrophic H2 consumer? This experiment was designed into two parts. In the experiment, C. minuta was co-cultivated with M. smithii in the working chamber of the BES. There, we generated H2 at the cathode in addition to the H2 that C. minuta generated to grow M. smithii without substrate limitations. Again, we detected a drop in the ratio of n-butyrate to acetate production of C. minuta. The second part of Objective 2 was not performed. There, we planned the co-cultivation of both microbes in the same BES but separated from each other in two chambers, where C. minuta would grow at the anode and M. smithii at the cathode. The idea of this experiment was that C. minuta grows at the anode and produces H2, which was removed by oxidation. The resulting H+ protons migrate through an ion exchange membrane, which separates the anode and cathode, and get reduced to H2 at the cathode, where M. smithii grows. For Objective 3, we wanted to use the BES to enrich syntrophic H2-producing microbes from the human gut that hide from current lab cultivation approaches. Therefore, we cultivated a human fresh stool sample in the BES under H2 removal conditions and detected the major fermentation products of the human gut microbiota and an enormous amount of biofilm formation. Because we detected fluctuation in the production profile of the SCCs and gasses, such as H2 and CO2, we assume that microbial composition changed during the cultivation period. The microbial community's evaluation and statistical analysis were done using the software tool “MMonitor” from Timo Lucas1 and the group of Prof. Daniel Huson1. Unfortunately, the final correlation between the composition of the microbiota and the detected metabolites in relation to H2 removal by the BES was not performed out until the end of my PhD. Future applications of the BES include isolating fastidious species requiring an H2 sink for growth. The outcomes of this study are essential to developing an isolation approach for gut microbes without requiring a microbial (syntrophic) partner. Culturing C. minuta and the entire human gut microbiota in the BES will help further technical BES development and support our ultimate goal of understanding human gut microbes better. Therefore, future applications of the BES include isolating particular strains requiring an H2 sink for growth. This approach could be used to enrich those host microbes that are highly prevalent, heritable, and health-associated bacteria without the need for drug treatment or fecal transplantation to cure intestinal diseases.
microbiota