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The term "normal microbial flora" denotes the population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons. The microorganisms that live inside and on humans (now referred to as the normal microbiota) are estimated to outnumber human somatic and germ cells by a factor of 10. The genomes of these microbial symbionts are collectively defined as the microbiome. Research has shown that the "normal microbiota" provides a first line of defense against microbial pathogens, assist in digestion, play a role in toxin degradation, and contribute to maturation of the immune system. Shifts in the normal microbiota or stimulation of inflammation by these commensals may cause diseases such as inflammatory bowel disease.

In a broad attempt to understand the role played by resident microbial ecosystems in human health and disease, in 2007, the National Institutes of Health launched the Human Microbiome Project. One of the main goals of this project is to understand the range of human genetic and physiologic diversity, the microbiome, and the factors that influence the distribution and evolution of the constituent microorganisms. One aspect of this project involves having several research groups simultaneously embark upon surveying the microbial communities on human skin and in mucosal areas such as the mouth, esophagus, stomach, colon, and vagina using small-subunit (16S) ribosomal RNA gene sequencing. Among the questions that will be addressed by this project are How stable and resilient is an individual's microbiota throughout one day and during his or her lifespan? How similar are the microbiomes between members of a family or members of a community or across communities in different environments? Do all humans have an identifiable "core" microbiome, and if so, how is it acquired and transmitted? What affects the genetic diversity of the microbiome, and how does this diversity affect adaptation by the microorganisms and the host to markedly different lifestyles and to various physiological or pathophysiological states? Numerous observations have already been made. For example, it has been determined that there are large differences among individuals in terms of the numbers and types of species of microorganisms inhabiting the colon and that obesity may be correlated with the types of microbes involved in specific metabolic pathways in the gastrointestinal tract. Readers should be aware that this field is rapidly evolving, and our understanding of the human microbiota will necessarily change as more information about resident microbial communities becomes available through the Human Microbiome Project.

The skin and mucous membranes always harbor a variety of microorganisms that can be arranged into two groups: (1) the resident microbiota consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it promptly reestablishes itself; and (2) the transient microbiota consists of nonpathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks. The transient microbiota is derived from the environment, does not produce disease, and does ...

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