It is estimated that 500 to 1,000 species of bacteria live in the human gut Bacterial cells are much smaller than human cells, and it is often said that there are at least ten times as many bacteria as human cells in the body, although the actual ratio has not been strictly defined.he human microbiota is the aggregate of microorganisms, a microbiome that resides on the surface and in deep layers of skin (including in mammary glands), in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts.