There is a whole world inside the human digestive tract, a vast and diverse collection of approximately 500, though possibly as many as 1000, species of bacteria and other microbes, working in harmony with each other and with their human host to digest and metabolize food. These microbes are called "gut microflora," or, collectively, "gut microbiota". It is a mutually beneficial relationship; the bacteria colonizing the organs get a place to live, while that place&mash;the human&mash;gets a world of possibilities for digesting food that the intestines alone are incapable of, such as digesting certain types of complex carbohydrates.
Collectively, gut flora weigh around two pounds. The composition of the gut microbiota is heavily influenced by location and diet. Where someone lives and where they grew up plays a large role in what sort of microflora will live in their intestines, though about a third of the microbiota appears be the same, substantially, in all humans, regardless of where and how they live.
Diet also plays a role; the balance of carbohydrates, proteins, and other components of food will gradually determine the mix of types of microbes living in the gut. This means, among other things, that the claim occasionally heard from vegetarians that the human body is not equipped to digest meat very will is actually true&mash;but only for vegetarians, who shift toward a gut microbiota balance optimized for plants. This is also why it’s possible to change the makeup of gut microbiota with probiotic supplements, or with probiotic foods such as yogurt. These have the effect of introducing microbes into the gut that may not normally be found there; even if these microbes don’t take over, they will change the ratios of the main ones already present.
While gut microflora are harmless as long as they are where they belong, they can cause serious health problems if they are moved to a different part of the body, or if they grow beyond their usual territory. This can happen as a result of antibiotics, which can lead to overproduction of some types of microflora by killing off other kinds, or by physical damage to the lining of the intestines