Interesting is that the variations on the natural flora vary greatly from individual to individual based upon diet, lifestyle, and environment.
In addition, there seems to be a correlation between gut bacteria and weight as well. Not the be all end all mind you... as it affects efficiency in digestion, absorption, utilization and the manufacture of vitamins, enzymes, etc. etc.
In general, we should be able to produce what we need by eating a whole foods, natural diet. However the current state of food, and the indiscriminate use of Antibiotics and medicines in general has thrown a huge curve ball into all of that.