Microflora, or gut flora, are the microorganisms that normally live in the digestive tract and perform a number of useful functions for their hosts.

The average human body, consisting of about ten trillion cells (10,000,000,000,000),and has about ten times that number of microorganisms in the gut. In fact, bacteria make up most of the flora in the colon. Somewhere between 300 and 1000 different species live in the gut, with most estimates at about 500.

Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather is a mutualistic, symbiotic relationship.

Though people can survive with no gut flora, the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful species,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.

References

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