Gut flora consists of a complex of microorganism species that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context gut is synonymous with intestinal, and flora with microbiota and microflora; the word microbiome is also in use. Gut flora's primary benefit to the host is the gleaning of energy from the fermentation of undigested carbohydrates and the subsequent absorption of short chain fatty acids. The most important of these are butyrates, metabolised by the colonic epithelium; propionates by the liver; and acetates by the muscle tissue. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolising bile acids, sterols and xenobiotics. 
The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than the total number of human cells in the body. The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a "forgotten" organ. It is estimated that these gut flora have around a hundred times as many genes in aggregate as there are in the human genome.
Bacteria make up most of the flora in the colon and up to 60% of the dry mass of feces. Somewhere between 300 and 1000 different species live in the gut, with most estimates at about 500. However, it is probable that 99% of the bacteria come from about 30 or 40 species. Fungi and protozoa also make up a part of the gut flora, but little is known about their activities.
Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship. Though people can survive without gut flora, the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful, pathogenic bacteria, regulating the development of the gut, producing vitamins for the host (such as biotin and vitamin K), and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease by producing infection or increasing cancer risk for the host.
Over 99% of the bacteria in the gut are anaerobes, but in the cecum, aerobic bacteria reach high densities. - wikipedia