Metabolic diversity in commensal protists regulates intestinal immunity and trans-kingdom competition 2023 Gerrick et al

Highlights

• Metabolic diversity in protists drives convergent Th1/Th17, but divergent Th2, immunity
• Protists’ preference for fiber or mucus shapes trans-kingdom competition with bacteria
• During fiber starvation, bacteria outcompete Tmu for the mucus niche
• Dietary fiber composition can modulate Tmu-induced type 2 immunity

Summary

The microbiota influences intestinal health and physiology, yet the contributions of commensal protists to the gut environment have been largely overlooked. Here, we discover human- and rodent-associated parabasalid protists, revealing substantial diversity and prevalence in nonindustrialized human populations. Genomic and metabolomic analyses of murine parabasalids from the genus Tritrichomonas revealed species-level differences in excretion of the metabolite succinate, which results in distinct small intestinal immune responses. Metabolic differences between Tritrichomonas species also determine their ecological niche within the microbiota.

By manipulating dietary fibers and developing in vitro protist culture, we show that different Tritrichomonas species prefer dietary polysaccharides or mucus glycans. These polysaccharide preferences drive trans-kingdom competition with specific commensal bacteria, which affects intestinal immunity in a diet-dependent manner. Our findings reveal unappreciated diversity in commensal parabasalids, elucidate differences in commensal protist metabolism, and suggest how dietary interventions could regulate their impact on gut health.

Paywall, https://www.cell.com/cell/fulltext/S0092-8674(23)01271-0

 

This group of bizarre gut microbes is unexpectedly complex

"Alongside the bacteria, fungi and other organisms living in our guts are single-celled, nucleus-bearing microorganisms called protists. A study in mice — whose protists are related to some found in humans — now shows how these under-studied organisms’ food preferences shape competition with other organisms in the microbiome and affect their host’s immune responses."

"Past research has shown that gut protists can activate immune responses. In mice, for example, the single-celled organism Tritrichomonas musculis excretes the molecule succinate, which kicks off an immune response in the small intestine called type 2 immunity2. The protist also boosts the number of immune cells called T helper 1 (TH1)3 and T helper 17 (TH17)4 — which send signals to other immune cells — in the colon. But little is known about gut-protist diversity, their metabolism or how they interact with other microbes."

"Experiments comparing the species showed that, like T. musculis, T. casperi triggers the production of TH1 and TH17 cells in the gut. But the newly described species doesn’t excrete succinate and is the only mouse gut protist in the genus Howitt has ever encountered that doesn’t activate type 2 immunity in the small intestine. “That was the first thing that really got me to sit up and take notice,” he says."

https://www.nature.com/articles/d41586-023-03990-8

 

The gut certainly is complex. I apparently lost an important strain, for no apparent reason, resulting in fibre intolerance. After a year of that, I got lucky and a strain in a common commercial probiotic capsule (14 strains) fixed the problem. Maybe that strain is the only one releasing a certain chemical, or maybe it was part of a complex interaction involving multiple strains of microbes competing for food and territory. Much more research is needed into understanding gut microbiome/body interaction.