Role of Vitamin B12 in sustaining trophic interactions between human gut symbionts

The aim of the proposal is to provide molecular and biochemical detail on the different forms of vitamin B12 (VitB12) that can be synthesised or utilised by gut bacteria in the gastrointestinal lining and understand how bacteria are able to participate in this nutrient provisioning and sharing. This fundamental knowledge is required to develop adapted nutritional approaches targeting gut microbes for the benefit of human health.

Trillions of bacteria live in our large intestine (the gut microbiota) where they play a vital role in maintaining good health. Bacteria in complex communities share resource and one of the key molecules that is communally distributed in such ecosystems is VitB12, a molecule that is made by only relatively few bacteria but utilised by most. Bacteria can make a range of up to 15 different VitB12-like molecules that collectively are called cobamides. Cobamides are expensive for the bacteria to make, requiring a complex biochemical pathway of around 30 enzymes for their biogenesis, but once made they provide the host with significant metabolic advantages. However, it is not clear to what extent these VitB12 variants or analogues are active across gut bacterial strains.

With the gut microbiota now being linked to so many different health conditions, there is a growing interest in developing nutritional strategies that look to alter the balance of microbes to improve health. This could be through dietary supplement or 'probiotic' approaches. Bacteria living in the gut lining (mucus-associated bacteria), close to our body, are particularly prone to affect or respond to changes in our health status. However, it is not usually known how to modulate these bacteria.

We previously showed that human strains of Lactobacillus reuteri are able to produce ViB12. In the work leading to this proposal, we demonstrated that Ruminoccus gnavus, a keystone mucus colonising bacterium in humans, requires VitB12 to grow but that, in the absence of VitB12, the presence of L. reuteri is also able to promote its growth. R. gnavus has been implicated as a key modulator of human health and there is therefore strong interest from academics and clinicians in better understanding the biology and requirements of this organism and how we can modulate it to improve human health. To address this mutualism and VitB12 requirement, we will determine the type of VitB12 forms required to sustain the growth of R. gnavus. We will identify the component parts in the bacteria that are required for the bacteria to utilise VitB12. By using anaerobic fermentation facilities and models to mimic the human colon we will determine the ability of VitB12-producers such as L. reuteri and/or VitB12 analogues to modulate VitB12-requirers such as R. gnavus and assess the impact of the treatments on the gut microbial community. We will then investigate the modulation of mucus-associated strains by investigating similar effects in vivo using mouse models harbouring a human gut microbiota. This will also inform on the impact of a 'probiotic' approach (in situ-production of VitB12) versus supplementation with specific VitB12 analogues.

Together, this collaborative proposal will provide long-sought information on the basic question of how bacteria have evolved their complex trade-offs by sharing metabolic burden. Furthermore, the research will gain and exploit fundamental knowledge on how VitB12 can lead to novel strategies for manipulating microbial composition in the gut for the benefit of human health.

Vitamin B12 (VitB12) plays a pivotal role in modulating and maintaining diverse microbial communities by acting as a shared resource that promotes networks of metabolically interdependent organisms. The aim of this project is to determine the role of VitB12 (and related cobamide molecules) in sustaining these trophic interactions between human gut symbionts and test the hypothesis that VitB12 can be used for the modulation of mucus-associated bacteria in the gut.

We will first decipher the mechanisms underpinning VitB12-mediated trophic interactions between two of the major gut symbionts, Lactobacillus reuteri and Ruminococcus gnavus, focusing on VitB12 biosynthetic/uptake and metabolism pathways, respectively. We will identify the VitB12 analogues sustaining R. gnavus growth and then exploit this knowledge to devise treatments for modulating the gut bacteria in complex microbial community in vitro and in vivo. Specifically, we will (1) investigate VitB12 cross-feeding between L. reuteri and R. gnavus strains (2) decipher the metabolic pathway for VitB12 uptake and utilisation by R. gnavus and (3) determine the impact of VitB12 on modulation of the human gut microbial community as well as test the effect of VitB12 on mucus-associated bacteria in vivo. Our multidisciplinary approach combines gut microbiology and chemical biology underpinned by expertise in advanced analytical and microscopy approaches, bacteria engineering, next-generation sequencing, in vitro fermentation models and humanised gnotobiotic mice.

This project aligns perfectly with the respective expertise of Prof N Juge (microbiology of gut bacteria-mucus adaptation) and Prof M Warren (biochemistry and synthesis of VitB12). The outcomes of the project will provide an enhanced understanding of how VitB12 can lead to novel strategies for manipulating mucus-associated microbiota in the gut for the benefit of human health.