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This article is about the microorganisms. For the genus, see Bacterium (genus). For other uses, see Bacteria (disambiguation).
Temporal range: Archean or earlier – Recent
Scanning electron micrograph of Escherichia coli bacilli
Woese et al., 1990
Gram positive / no outer membrane
Tenericutes (no wall)
Gram negative / outer membrane present
Bacteroidetes/Fibrobacteres–Chlorobi (FCB group)
Planctomycetes–Verrucomicrobia/Chlamydiae (PVC group)
Unknown / ungrouped
Eubacteria Woese & Fox, 1977
Bacteria (i/bækˈtɪəriə/; singular: bacterium) constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. They are also known to have flourished in manned spacecraft.
There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are approximately 5×1030 bacteria on Earth, forming a biomass which exceeds that of all plants and animals....