Inside our gut lives a major population of bacteria, called our "commensal flora."
Our gut bacteria play a critical role in keeping us healthy by:
making us vitamins
breaking down complex foods
strengthen the gut barrier
In return, our body provides a stable environment for the bacteria to live along with a steady nutrient supply. When we eat- we are also feeding our gut bacteria.
There are different types of bacteria found along the gastrointestinal tract:
In the stomach, only a few bacterial species are able to colonize, due to the low pH from hydrochloric acid and the activity of proteolytic enzymes (1).
In the small intestine, there are more bacteria, with approximately 10^7 bacteria/gram of luminal material that can be detected (2). The small intestine contains a higher relative abundance of members of the Actinobacteria and Streptococcaceae phyla (3). The small intestine of infants has also been shown to contain segmented filamentous bacteria (SFB), gram-positive, spore-forming bacteria that can specifically induce Th17 cells (4).
In the large intestine, the slower transit times and the high supply of nutrients from undigested food allows for the highest density and abundance of bacteria (5).
The healthy adult colon contains more than 100 trillion (10^14) bacteria that comprise many different strains (6) and the total number of genes derived from this diverse microbiome may exceed that of the entire human genome by at least 100-fold (7).
In human adults, the colon is characterized by the predominance of two phyla, Bacteroidetes and Firmicutes (which include Clostridium and Lactobacillus microbes), with minor contributions from the Proteobacteria and Actinobacteria (including Bifidobacteria) phyla (8).
The makeup of our commensal flora is largely determined by environmental factors, for instance vaginal birth vs. caesarian section, breast milk vs. formula feeding and eating an animal vs. plant-based diet have all been shown to change the gut bacteria composition (9).
Synthesis of Vitamins
Vitamins are essential for human health and must be provided from outside sources, including diet and supplementation.
The bacteria in the human colon has been shown to make many different vitamins (10), including:
vitamin K (menaquinones)
Vitamin B1 (thiamine)
Vitamin B2 (riboflavin)
Vitamin B3 (nicotinic acid)
Vitamin B5 (pantothenic acid)
Vitamin B6 (pyridoxine)
Vitamin B7 (biotin)
Vitamin B9 (folate)
Vitamin B12 (cobalamin)
Unlike dietary vitamins, which are mainly absorbed in the proximal part of the small intestine, the uptake of bacteria-made vitamins can occur in the colon (11).
Colonocytes appear to be able to absorb menaquinones, thiamine, riboflavin, pantothenic acid, biotin and folate, indicating that the microbiota-produced vitamins may contribute to systemic vitamin levels (11, 12).
The gut microbiota can makes enzymes that can break down complex molecules, such as plant polysaccharides and fibers. The fermentation of dietary components results in the production of short chain fatty acids (SCFAs), (e.g., acetic, lactic, propionic, and butyric acids), branched chain fatty acids (e.g., isobutyric, isovaleric, and 2-methylbutyric acids), carbon monoxide, ammonia, amines and several other end products.
These fermentation products affect the gut environment and the host’s health, acting as energy sources, regulators of gene expression and cell differentiation, as well as anti-inflammatory agents.
Maintenance of Intestinal Barrier
Commensal bacteria and probiotics have been shown to promote intestinal barrier integrity both in vitro and in vivo by targeting the expression and distribution of tight junction proteins (13).
Host Defense against Pathogens
The gut microbiota can also effectively defend the host against enteric bacterial infections by competing with pathogens for habitats and nutrients (14). As such, host-microbe interactions are essential for host resistance to pathogenic infections, gut development, and epithelial homeostasis (15).