The human microbiome, representing the collection of all microbes
that live on and within a human being, is composed of 10
times more cells than human cells (Eckburg et al. 2005;Walter and
Ley 2011). These naturally occurring microbes, particularly those
that reside in the human gut, are known to provide humans with
crucial metabolic functions. They allow harvesting and storing
energy from various dietary products, influence the development
of the immune system, and protect from colonization by pathogens
(Hooper et al. 2002; Dethlefsen et al. 2007). In a recent study
by the MetaHIT Consortium (Metagenomics of Human Intestinal
Tract) (Qin et al. 2010), DNA from fecal samples of 124 Europeans
was sequenced to generate deep coverage of the human gut
microbiome. Analysis of the assembled DNA fragments showed
that the human gut microbial gene set is 150 times larger than the
human gene complement (Qin et al. 2010).