Temporal range: Archean or earlier – Recent
Scanning electron micrograph of Escherichia coli bacilli
gram positive/no outer membrane
Tenericutes (no wall)
gram negative/outer membrane present
Fibrobacteres–Chlorobi/Bacteroidetes (FCB group)
Planctomycetes–Verrucomicrobia/Chlamydiae (PVC group)
Bacteria (/bækˈtɪəriə/ ( listen); singular: bacterium) are a large domain of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×1030) bacteria on Earth, forming a biomass on Earth, which exceeds that of all plants and animals. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. However, most bacteria have not been characterised, and only about half of the phyla of bacteria have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.
There are approximately...