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Many bacterial species exist simply as single cells, others associate in characteristic patterns: Neisseria form diploids (pairs), Streptococcus form chains, and Staphylococcus group together in "bunch of grapes" clusters.
Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust.These multicellular structures are often only seen in certain conditions.For example, when starved of amino acids, Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells.Acidobacteria Actinobacteria Aquificae Armatimonadetes Bacteroidetes Caldiserica Chlamydiae Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Elusimicrobia Fibrobacteres Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Synergistetes Tenericutes Thermodesulfobacteria Thermotogae Verrucomicrobia ; common noun bacteria, 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.The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago.
For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.
Bacteria also live in symbiotic and parasitic relationships with plants and animals.
Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory.
The study of bacteria is known as bacteriology, a branch of microbiology.
There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water.
This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants.