A few days ago, a fun article[i] appeared on Gut Microbiota for Health which listed their most popular articles in 2018. Contrary me: I looked through all 10 and found that the last one on the list was way the most interesting.
In your small intestine, there is lymph tissue (GALT = gut-associated lymphoid tissue), where your cells interact with both gut microbes and immune cells. This is where the decision is made whether or not you will tolerate something in the gut (or attack it as an invader). In the GALT, gut bacteria help make an immune cell that “…enables its host to function normally by tolerating harmless substances from the outside.”
Researchers from Washington University in St. Louis’ School of Medicine wanted to find out specifically which commensal bacteria was responsible for the production of this immune cell, known as DP IEL (double-positive intraepithelial lymphocyte), which help the body to distinguish self from non-self and bad from good, regulating inflammatory responses accordingly.[ii] These cells are absent in germ-free mice, so it was already established that commensal bacteria were crucial in its production…but which specific bacteria were responsible was unknown.
By giving germ-free mice different strains of Lactobacillus and Bacteroides, these scientists were able to establish which led to increased levels of DP IEL, and they discovered two crucially important things. Firstly, the essential amino acid (meaning that you must eat it to get it – your body cannot produce it on its own), L-tryptophan, is necessary to in the process of creating DP IEL. The higher the level of tryptophan fed to the mice, the higher their level of DP IELs. (Foods that are high in tryptophan include turkey, salmon, eggs, cheese, nuts, seeds.) Secondly, they discovered that the probiotic bacteria, L.reuteri, was also crucial in its production. Interestingly, L.reuteri’s effectiveness in the production process was enhanced when they added other beneficial species to the mix.[iii]
Holy cow! How astounding is that? I have written before about how essential L.reuteri seems to be in illnesses like autism and PTSD, both of which are associated with alterations in the microbiome and high levels of inflammation. And by the way, tryptophan metabolism, which is the key in the formation of messenger molecules like serotonin, has long been considered suspect in various “mental” illnesses. This tryptophan/L.reuteri combo has not as yet been tested in humans, but I am sure that’s only a matter of time. In the meantime, since we already know it has a positive effect on autism, depression and anxiety disorders, adding some to the diet is certainly not going to hurt.
[iii] Cervantes-Barragan L, Chai JN, Tianero MD, DiLuccia B, Ahern PP, Merriman J, Cortez VS, Caparon MG, Donia MS, Gilfillan S, Cella M, Gordon JI, Hsieh C-S, Colonna M. Lactobacillus reuteri induces gut intraepithelial CD4 CD8 alpha alpha T cells. Science. 2017; 357(6353): 806-810. doi: 10.1126/science.aah5825.