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.
A small number of other unusual shapes have been described, such as star-shaped bacteria.
This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.
In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, and the recovery of gold, palladium, copper and other metals in the mining sector, 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.
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.The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year, mostly in sub-Saharan Africa.In developed countries, antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem.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 were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes.