[esm]

Gut bacteria have many functions, and there are likely more out there to be discovered than we know about currently. One of the functions that I find interesting is that bacteria help to synthesize vitamin K, which is required by the liver to build proteins that allow blood to clot. Neonates do not have the gut flora to synthesize vitamin K at birth, which is why they get a vitamin K shot after delivery.

Normal gut bacteria also coat the GI tract, preventing pathogenic bacteria from sticking to the surface of the intestines. This is part of the reason babies under 1 should not eat raw honey (which has C. botulinum spores that can stick to the intestines and produce botulinum toxin), while older kids and adults have no trouble with it.

Along the same lines, a hot (though, admittedly disgusting) area of research in medicine now is fecal transplant for people with a particularly aggressive form of colitis. I believe that the research on glut flora transplantation is also being expanded to weight loss, but I'm not very familiar with the studies.