The right answer is bacteria.
The intestinal microflora is in permanent interrelation with food and the human organism, the whole constituting a complex ecosystem. Essential to the proper functioning of the body, the intestinal microflora should be better known thanks to new exploration techniques.
In humans intestinal microflora contains about 100 000 billion bacteria belonging to more than 400 different species. It is a considerable biomass whose activity results in the in vivo production of enzymes and metabolites.