Gut bacteria. Scanning electron micrograph (SEM) of various bacteria found in a sample from a human small intestine. The human intestine microbiome primary benefit is the uptake of energy from the fermentation of undigested carbohydrates and the absorption of short chain fatty acids. Research has shown that the relationship between intestinal flora and humans is not just commensal (a non-harmful coexistence), but rather a mutualistic relationship. The human body carries about 100 trillion microbes in the intestinal tract, a number 10 times greater than the total number of human cells in the body. Magnification: x15000 when printed 10 centimetres wide.