[muawiyah]

Aren't there several hundred different bacteria and yeasts in the human gut that seem to have some sort of part to play in our normal processes?

Supposedly they have a combined genetic diversity many times greater than our own.

[aruanan]

Aren't there several hundred different bacteria and yeasts in the human gut that seem to have some sort of part to play in our normal processes? Supposedly they have a combined genetic diversity many times greater than our own.

Bacteroides is the most common bacteria in the human gut with various others, Wikipedia says about 500 species in total. Their cells outnumber ours up to 10:1.

This Wikipedia entry is a pretty good summary of what I've read in my textbooks:

Research suggests that the relationship between gut flora[9] and humans is not merely commensal (a non-harmful coexistence), but rather is a mutualistic, symbiotic relationship.[3] Though people can survive with no gut flora,[4] the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful species,[2] 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 causing infection or increasing cancer risk for the host.[2][6]