Meet the microbe family

Microbes fall into five big groups. Four of them are made of cells, and one is not.

• Bacteria: single cells, usually 0.5 to 5 µm, with no nucleus. DNA floats in the cell. They live almost everywhere: soil, water, your gut and your skin.
• Archaea: single cells that look like bacteria under a microscope but are a separate branch of life. Many live in extreme places such as hot springs and very salty lakes, but they are also found in soil, oceans and the human gut. No archaeon has been clearly shown to cause an infectious disease in humans.
• Fungi: cells with a nucleus. Yeasts are single cells, moulds grow as threads. Yeast makes bread rise.
• Protists: a mixed group of mostly single-celled organisms with a nucleus, such as amoebae and Paramecium.
• Viruses: not cells at all. A virus is a package of genes in a protein coat. It cannot make copies of itself without borrowing a living cell.

Biologists sort all cellular life into three domains: Bacteria, Archaea and Eukarya (organisms whose cells have a nucleus, including fungi, protists, plants and animals). Viruses are not part of these three domains because they are not cells.

How many bacteria live in and on you? For many years people said that bacterial cells outnumber human cells ten to one. That figure was a rough guess and it did not hold up. A careful 2016 estimate by Ron Sender, Shai Fuchs and Ron Milo, for a "reference" 70 kg adult man, found about 3.8 × 10¹³ bacteria and about 3.0 × 10¹³ human cells. That is closer to one bacterium for every human cell, and the exact ratio changes from person to person and from day to day.