Bacteria; common noun bacteria , singular bacterium are a type of biological cell . They 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 biosphere of the earth's crust . Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent 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.
Nearly all animal life is dependent on bacteria for survival as only bacteria and some archaea possess the genes and enzymes necessary to synthesize vitamin B 12 , also known as cobalamin , and provide it through the food chain. Vitamin B 12 is a water-soluble vitamin that is involved in the metabolism of every cell of the human body. It is a cofactor in DNA synthesis , and in both fatty acid and amino acid metabolism . It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin. 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 5×10 30 bacteria on Earth, forming a biomass which exceeds that of all plants and animals. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere . The nutrient cycle includes the decomposition of dead bodies ; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps , extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane , to energy.
