Organism or group
Bacteria
Small single-celled organisms without a nucleus, found almost everywhere on Earth. Most bacteria in the human body are harmless, and some are helpful.
Microbiome
The communities of microbes living in and on a body. A 2016 estimate put a reference adult's bacteria at about 38 trillion, similar to the number of human cells.Source: Sender et al., 2016Source: HMP Consortium, 2012Source: NIH
Idea or model
How Microbiome connects
Microbiome contains
Microbiome is related to
Idea or model
Cells of the human body
An adult has roughly 30 to 37 trillion human cells, and about 90 percent of them belong to the blood-forming lineage.
Studied in
Your progress
Sources & further reading
The explanations on this page are our own summaries of these sources. Follow a link to read the original; if anything here disagrees with it, the original wins. How we choose and check sources.
Revised estimates for the number of human and bacteria cells in the body (opens the original in a new tab)
Sender R, Fuchs S, Milo R (2016). PLOS Biology 14:e1002533.
About 3.0 × 10^13 human cells and 3.8 × 10^13 bacteria in a 70 kg reference adult.
Peer-reviewedPrimary researchStructure, function and diversity of the healthy human microbiome (opens the original in a new tab)
Human Microbiome Project Consortium (2012). Nature 486:207-214.
Peer-reviewedPrimary researchHuman Microbiome Project (HMP) (opens the original in a new tab)
NIH Common Fund. commonfund.nih.gov. National Institutes of Health.
ReferenceGovernment sourceEducational