Human microbiome

The human microbiota is the aggregate of microorganisms, a microbiome that resides on or within a number of tissues and biofluids, including the skin, mammary glands, placenta, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, conjunctiva, and gastrointestinal tracts. They include bacteria, fungi, and archaea. Micro-animals which live on the human body are excluded. The human microbiome refers to their genomes.

Humans are colonized by many microorganisms; the traditional estimate was that humans live with ten times more non-human cells than human cells; more recent estimates have lowered that to 3:1 and even to approximately the same number; all the numbers are estimates. Some microbiota that colonize humans have not merely a commensal (a non-harmful coexistence), but rather a mutualistic relationship with their human hosts. Conversely, some non-pathogenic microbiota can harm human hosts via the metabolites that they produce, like trimethylamine. Certain microbiota perform tasks that are known to be useful for the human host; for most, the role is not well understood. Those that are expected to be present, and that under normal circumstances do not cause disease, are deemed normal flora or normal microbiota.

The Human Microbiome Project took on the project of sequencing the genome of the human microbiota, focusing particularly on the microbiota that normally inhabit the skin, mouth, nose, digestive tract, and vagina. It reached a milestone in 2012 when it published initial results.

Though widely known as flora or microflora, this is a misnomer in technical terms, since the word root flora pertains to plants, and biota refers to the total collection of organisms in a particular ecosystem. Recently, the more appropriate term microbiota is applied, though its use has not eclipsed the entrenched use and recognition of flora with regard to bacteria and other microorganisms. Both terms are being used in different literature.

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