Research over the last decade and more has shown that the human body is not nearly as self-sufficient as we always thought – instead it is a complex ecosystem, containing trillions of bacteria and other microorganisms that inhabit our skin, genital areas, mouth and especially our gut. Bacterial cells (friendly bacteria) in the human body outnumber human cells 10 to one! This microbiome – this mixed community of bacteria and microorganisms and the genes they contain – is of no threat to us, but rather offers vital help with basic physiological processes, like digestion, growth and self-defense.
Source : Scientific American May 2012