Explore the unseen organisms that live on our bodies.

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  • Microbes are organisms that cannot be seen with the naked eye. They include bacteria, archaea, protists, viruses, prions, and fungi. They account for most of the diversity of life on Earth. Far more microbes inhabit a single person’s body than there are people on the entire planet. Microbes also dominate in terms of their ability to live and thrive in extreme environments, including clouds, deep-sea volcanoes, springs, ice caps, and animal intestines.

    Most animals, including humans, exhibit numerous associations with microbes. Our "human microbiome" includes our skin, hair, mouth, and gut.

    Humans typically associate microbes with disease. Most of our human microbiome, however, is neutral or even beneficial. Microbes help digest food, absorb nutrients, and out-compete harmful bacteria in the intestines. They produce vitamins and proteins that human genes cannot produce. They prevent the growth of harmful skin bacteria and further aid the immune system in fighting infections and diseases throughout the human body. 

    • The microbes that inhabit a single person outnumber the human cells ten to one and can weigh approximately 1.4 kilograms (3 pounds).
    • In addition to the human microbiome, microbes live in plants, soil, oceans, and the atmosphere. These microbiomes maintain healthy function of these diverse ecosystems, influencing human health, climate change, food security, and other factors. 
    • Escherichia coli is a bacterium that is one of the most well-studied microbes in our human microbiome. E.coli can cause severe intestinal illnesses, but also plays an important role in providing humans with vitamin K.
    • The air around you may be as distinctive as a fingerprint. The human body emits bacteria into the air, and the resulting microbial cloud may be used to uniquely identify the person.
  • Term Part of Speech Definition Encyclopedic Entry
    archaea Plural Noun

    (singular: archaeon) a group of tiny organisms often living in extreme environments, such as ocean vents and salt lakes.

    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    digest Verb

    to convert food into nutrients that can be absorbed.

    disease Noun

    a harmful condition of a body part or organ.

    fungi Plural Noun

    (singular: fungus) organisms that survive by decomposing and absorbing nutrients in organic material such as soil or dead organisms.

    gene Noun

    part of DNA that is the basic unit of heredity.

    human microbiome Noun

    collection of microorganisms that reside on the surface and in deep layers of the skin, mouth, eyes, and gut of living human beings. The human biome includes bacteria, fungi, and archaea, but does not include microscopic animals.

    ice cap Noun

    area of fewer than 50,000 square kilometers (19,000 square miles) covered by ice.

    Encyclopedic Entry: ice cap
    immune system Noun

    network of chemicals and organs that protects the body from disease.

    intestine Noun

    tubular system in the human digestive system, which regulates elimination of waste products from the body.

    microbe Noun

    tiny organism, usually a bacterium.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    organism Noun

    living or once-living thing.

    prion Noun

    microscopic particle of protein that is thought to be able to self-replicate and to be the agent of infection in a variety of diseases of the nervous system.

    protein Noun

    one of many complex compounds, made of chains of amino acids, that make up the majority of all cellular structures and are necessary for biological processes.

    protist Noun

    type of microscopic organism (not an animal, plant, or fungus). 

    spring Noun

    small flow of water flowing naturally from an underground water source.

    virus Noun

    pathogenic agent that lives and multiplies in a living cell.

    vitamin Noun

    chemical substance that is necessary for health.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano


National Science Foundation Mysteries of the Unseen World

This material is based in part upon work supported by the National Science Foundation under Grant No. DRL-0840250. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

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