About the Second Brain and Its Citizens
The Microbiota-Gut-Brain Axis
The human intestinal tract contains roughly as many bacterial cells as there are human cells in the entire body. It has been shown that some gut bacteria are able to produce neurotransmitters that can modulate the concentration of these chemicals in the brain.
In addition to that, recent research found a direct neural connection between the gut lumen and the brain through specialized epithelial cells, the so-called neuropods.  These are cells that, on one hand, create receptors towards the gut lumen capable of sensing a variety of different metabolites and, on the other hand, form synapses to neuronal cells. 
To draw the whole picture: Bacteria can produce metabolites which are sensed by neuropod cells. In turn, the gut can excrete compounds that can interact with the bacteria, thus forming a closed circle of communication.
A Movie Starring Bacteria
The aim of this work is to get more insights into the interaction between the brain and the gut microbiota by investigating their changes over time.
Longitudinal sampling of gut microbiome composition and brain function in a single individual will serve as data for this project. It consists of a collection of microbial DNA from stool samples, fMRI brain scans, and physiological and psychological measures that were collected within a period of two years. Physiological data were collected with the help of smart watches worn by the individual. Psychological data were
assessed with diaries, questionnaires, and cognitive tests. Hypothesis about the possible interaction of these variables will be formulated as a first step. Following that, the data will be processed and said hypothesis will be tested with the help of bioinformatic pipelines and statistical procedures. 
Possible findings could point towards the importance of considering the role of microbiota when investigating cognition. It could also show the relevance of targeting microbiota as a mean of psychological therapy.
Viewing the gut lumen as external to the body, the influence of the gut bacteria on the brain could have interesting implications for the extended mind theory (EMT).
Lastly, the insights might raise questions related to phenomenology: Do the sensing of bacteria and their metabolites constitute a distinct sensory modality which we yet must learn to interpret?
 M. M. Kaelberer et al., “A gut-brain neural circuit for nutrient sensory transduction,” Science, vol. 361, no. 6408, 2018, doi: 10.1126/science.aat5236.
 K. L. Buchanan et al., “The preference for sugar over sweetener depends on a gut sensor cell,” Nat Neurosci, vol. 25, no. 2, pp. 191–200, 2022, doi: 10.1038/s41593–021- 00982–7.
 A. Sorbie, R. Delgado Jiménez, and C. Benakis, “Increasing transparency and reproducibility in stroke-microbiota research: A toolbox for microbiota analysis,” iScience, vol. 25, no. 4, p. 103998, 2022, doi: 10.1016/j.isci.2022.103998.