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The human microbiome is the aggregate of all microbiota that reside on or within human tissues and biofluids along with the corresponding anatomical sites in which they reside, including the gastrointestinal tract, skin, mammary glands, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, ocular surface, and the biliary tract.

Types of human microbiota include bacteria, archaea, fungi, protists, and viruses. Though micro-animals can also live on the human body, they are typically excluded from this definition. In the context of genomics, the term human microbiome is sometimes used to refer to the collective genomes of resident microorganisms; however, the term human metagenome has the same meaning. The human body hosts many microorganisms, with approximately the same order of magnitude of non-human cells as human cells. Some microorganisms that humans host are commensal, meaning they co-exist without harming or benefiting humans; others have a mutualistic relationship with their human hosts, where both parties benefit. Conversely, some non-pathogenic microorganisms can harm human hosts via the metabolites they produce, like trimethylamine, which the human body converts to trimethylamine N-oxide via FMO3-mediated oxidation. Certain microorganisms perform tasks that are known to be useful to the human host, but the role of most of them is not well understood. Those that are expected to be present, and that under normal circumstances do not cause disease, are sometimes deemed normal flora or normal microbiota. The Human Microbiome Project (HMP) 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.

The number of bacterial cells in the human body is estimated to be around 38 trillion, while the estimate for human cells is around 30 to 40 trillion. The number of bacterial genes is estimated to be 2 million, 100 times the number of approximately 20,000 human genes.

The problem of elucidating the human microbiome is essentially identifying the numerous members of a diverse microbial community, which includes bacteria, eukaryotes, and viruses. One of the challenges that is present in human microbiome studies, but not in other metagenomic studies, is to avoid including the host DNA in the study. 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. It is known that the human microbiome (such as the gut microbiota) is highly variable both within a single subject and among different individuals, a phenomenon also observed in mice.

The Human Microbiome Project found that individuals host thousands of bacterial types, with different body sites having their own distinctive communities.

The composition of human gut microbiota changes over time, when the diet changes, and as overall health changes.

The environment present in the human mouth allows the growth of characteristic microorganisms found there.

Both metagenomic and epidemiological studies indicate vital roles for the human microbiome in preventing a wide range of diseases, from type 2 diabetes and obesity to inflammatory bowel disease, Parkinson's disease, and even mental health conditions like depression.

Alterations in the gut microbiome have also been reported in other inflammatory skin diseases.

Hygiene, probiotics, prebiotics, synbiotics, light therapy, microbiota transplants (fecal or skin), antibiotics, exercise, diet, breastfeeding, aging can change the human microbiome across various anatomical systems or regions such as skin and gut.

The human microbiome is transmitted between a mother and her children, as well as between people living in the same household.

A 2024 study suggests that gut microbiota capable of digesting cellulose can be found in the human microbiome, and they are less abundant in people living in industrialized societies.

Quick Facts

• The Human Microbiome Project (HMP) 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.
• In the context of genomics, the term human microbiome is sometimes used to refer to the collective genomes of resident microorganisms; however, the term human metagenome has the same meaning.
• The human body hosts many microorganisms, with approximately the same order of magnitude of non-human cells as human cells.
• Types of human microbiota include bacteria, archaea, fungi, protists, and viruses.
• Conversely, some non-pathogenic microorganisms can harm human hosts via the metabolites they produce, like trimethylamine, which the human body converts to trimethylamine N-oxide via FMO3-mediated oxidation.

Source material: Wikipedia - "Human microbiome". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.