Escherichia coli, one of the many species of bacteria present in the human gut

Gut flora consists of microorganisms that live in the digestive system of animals. It is the largest reservoir of microbes in the human microbiome.

The human body, consisting of about 100 trillion cells, carries about ten times as many microorganisms in the intestines. The metabolic activities performed by these bacteria resemble those of an organ, leading some to call gut flora a 'forgotten organ'. It is estimated that these gut flora have around 100 times as many genes in aggregate as there are in the human genome.