Biofilms as Human Gut Mycorrhizals

Are Biofilms Healthy Extensions of Intestinal Villi?

If soil is the stomach of the earth, then plant roots and mycorrizal fungi must be the intestines.

Mycorrhizal fungal hyphae extend from root hairs of plants into surrounding soil and enhance the uptake of phosphate and other nutritents.  Many plants cannot colonize new soil without taking their fungal partners with them.  It would seem obvious that the highly adapted human gut flora would include bacteria and fungi that actively communicate with intestinal epithelial cells.  Perhaps that communication includes both nutrients, e.g. hydrogen, ammonia, etc., vitamins and bacterial wall components, e.g. LPS.

Plants Sit and Mine Soil, Humans Mine Nutrients Passed through Their Gut

I want to try to give a plant’s view of human digestion.  Plants elaborate roots that branch repeatedly and the final extensions sprout hairs from individual epithelial cells.  Mycorrhizal fungal hyphae further extend the reach of the plant into the soil for nutrients. 

I think that a plant would look at us and see us stuffing soil/food into our mouths and watch it come out the other end.  It would then try to figure out where are roots are, i.e. how we absorb the water and minerals from our moving internal stream of soil.  The villi of the small intestines would look like root hairs, but where are the mycorrhizal fungi?  Another problem is that the soil keeps moving past the root hairs and would break off fungal hyphae extending into the soil.  Still another problem is the constant shedding of epithelial cells from the tips of the villi.  The plant would be perplexed, but closer inspection would reveal that biofilms could solve the problems.

Biofilms Coat the Intestinal Villi

Biofilms coating and perhaps spanning the villi of the small intestines may enhance the transport of nutrients into the villi.  This may be controversial and the biofilms may be more commonly limited to the smoother surface of the colon.  The point here is that biofilms may enhance the intestinal uptake of nutrients from food.  Biofilms may, therefore, be essential for health and extend the reach of the intestinal epithelial cells.

Bacterial Community Composition May Be Determined by Diet

A biofilm is composed of some type of linear polymer, such as DNA, heparan sulfate or bacterial acidic polysaccharides, with bacteria that bind to the polymer and to the intestinal epithelium.  Diet determines the bacterial composition of the biofilm.  Thus, the newborn starts without biofilms, gut development is finished by growth hormones in milk and a single species of Bifidobacteria excludes biofilm production, until solid food or formula initiates adult biofilms.  The bacteria in the biofilm depend on diet, so the biofilms can be either beneficial or pathogenic.

Communication within Biofilms and with the Intestines

The bacteria respond to the presence of other bacteria by quorum sensing, which involves release of small molecules that alter the gene expression of other bacteria in the community.  As a consequence, genes, e.g. antibiotic resistance, are exchanged and metabolism is altered.  This is how Klebsiella nitrogenase and hydrogen production is controlled.  The biofilm bacteria also produce compounds, e.g. vitamin D (?), that alter the behavior of the intestinal epithelial cells.  The intestines can respond with inflammation to recognized pathogen components or by triggering development of cells of the immune system.  The intestines are the home of most of the body’s immune cells.

Stimulation of Tregs

Helicobacter pylori adhering to the stomach lining increases the stomach’s quota of regulatory T cells that are involved in immunological tolerance.  Presumably, the supply of Tregs in the intestines is also regulated by biofilms.  Disruption of this system by chronic inflammation can deplete Tregs and lead to unrestrained immune attack that is observed as inflammatory bowel disease.  Thus, Crohn’s disease and ulcerative colitis may be triggered by damaged biofilms.