This article is about the microorganisms. For the genus, see Bacterium (genus). For other uses, see Bacteria (disambiguation).
| Bacteria Temporal range: Archean or earlier – Recent |
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| Scanning electron micrograph of Escherichia coli bacilli | |
| Scientific classification | |
| Domain: | Bacteria |
| Phyla[1] | |
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Actinobacteria (high-G+C) Firmicutes (low-G+C) Tenericutes (no wall)
Deinococcus-Thermus Fibrobacteres–Chlorobi/Bacteroidetes (FCB group) Fusobacteria Gemmatimonadetes Nitrospirae Planctomycetes–Verrucomicrobia/Chlamydiae (PVC group) Proteobacteria Spirochaetes Synergistetes
Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Dictyoglomi Thermodesulfobacteria Thermotogae |
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i/bækˈtɪəriə/; singular: bacterium) constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range 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 habitats on the planet. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,[2] and the deep portions of Earth's crust. Bacteria also live in plants, animals (see symbiosis), and have survived in space.[3]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,[4] forming a biomass that exceeds that of all plants and animals.[5] 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. In the biological communities surrounding hydrothermal vents and cold seeps, bacteria provide the nutrients needed to sustain life by converting dissolved compounds such as hydrogen sulphide and methane to energy. On 17 March 2013, researchers reported data that suggested bacterial life forms thrive in the Mariana Trench, the deepest spot on the Earth.[6][7] Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States.[6][8] According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."[6]
Most bacteria have not been characterised, and only about half of the phyla of bacteria have species that can be grown in the laboratory.[9] The study of bacteria is known as bacteriology, a branch of microbiology.
There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora.[10] The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and a few are beneficial. However, a few species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people a year, mostly in sub-Saharan Africa.[11] In developed countries, antibiotics are used to treat bacterial infections and in agriculture, so antibiotic resistance is becoming common. In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector,[12] as well as in biotechnology, and the manufacture of antibiotics and other chemicals.[13]
Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.[14]
Origin and early evolution
Further information: Timeline of evolution and Evolutionary history of life
The ancestors of modern bacteria were single-celled microorganisms that were the first forms of life
to appear on Earth, about 4 billion years ago. For about 3 billion
years, all organisms were microscopic, and bacteria and archaea were the
dominant forms of life.[18][19] Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology
prevents them from being used to examine the history of bacterial
evolution, or to date the time of origin of a particular bacterial
species. However, gene sequences can be used to reconstruct the
bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[20]Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from ancient bacteria entering into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.[21][22] This involved the engulfment by proto-eukaryotic cells of alpha-proteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.[23][24] This is known as secondary endosymbiosis.
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