<P> Aside from simply elucidating the composition of the human microbiome, one of the major questions involving the human microbiome is whether there is a "core", that is, whether there is a subset of the community that is shared among most humans . If there is a core, then it would be possible to associate certain community compositions with disease states, which is one of the goals of the Human Microbiome Project . It is known that the human microbiome is highly variable both within a single subject and among different individuals . For example, the gut microbiota of humans is markedly dissimilar among individuals, a phenomenon which is also observed in mice . </P> <P> On 13 June 2012, a major milestone of the Human Microbiome Project (HMP) was announced by the NIH director Francis Collins . The announcement was accompanied with a series of coordinated articles published in Nature and several journals in the Public Library of Science (PLoS) on the same day . By mapping the normal microbial make - up of healthy humans using genome sequencing techniques, the researchers of the HMP have created a reference database and the boundaries of normal microbial variation in humans . From 242 healthy U.S. volunteers, more than 5,000 samples were collected from tissues from 15 (men) to 18 (women) body sites such as mouth, nose, skin, lower intestine (stool), and vagina . All the DNA, human and microbial, were analyzed with DNA sequencing machines . The microbial genome data were extracted by identifying the bacterial specific ribosomal RNA, 16S rRNA . The researchers calculated that more than 10,000 microbial species occupy the human ecosystem and they have identified 81--99% of the genera . </P> <P> Populations of microbes (such as bacteria and yeasts) inhabit the skin and mucosal surfaces in various parts of the body . Their role forms part of normal, healthy human physiology, however if microbe numbers grow beyond their typical ranges (often due to a compromised immune system) or if microbes populate (such as through poor hygiene or injury) areas of the body normally not colonized or sterile (such as the blood, or the lower respiratory tract, or the abdominal cavity), disease can result (causing, respectively, bacteremia / sepsis, pneumonia, and peritonitis). </P> <P> The Human Microbiome Project found that individuals host thousands of bacterial types, different body sites having their own distinctive communities . Skin and vaginal sites showed smaller diversity than the mouth and gut, these showing the greatest richness . The bacterial makeup for a given site on a body varies from person to person, not only in type, but also in abundance . Bacteria of the same species found throughout the mouth are of multiple subtypes, preferring to inhabit distinctly different locations in the mouth . Even the enterotypes in the human gut, previously thought to be well understood, are from a broad spectrum of communities with blurred taxon boundaries . </P>

Types of bacteria found in the human body