Bacterium
Bacteria are a group of single-celled microorganisms. Typically, bacteria of several micrometers in length have various forms, some spherical, some spiral, some rod, some comma. Bacteria are present in every environment on earth. There are types that can grow in soil, in sea water, in the depths of the ocean, in the earth's crust, in the deep, in the intestines of animals, in acidic hot water springs, in radioactive wastes. Typically, the number of bacterial cells in a gram of soil is 40 million, and in a milliliter of fresh water is one million; collectively there are five non-million bacteria in the world, which constitute the majority of the world's biomass. Bacteria have a vital prescription for the recycling of food, and most important steps in the food cycle are bacterial-like, such as nitrogen fixation atmospheres. However, most of these bacteria are not yet identified and only about half of the bacterial strains have species that can be cultured in the laboratory. The science in which bacteria are investigated is bacteriology, a branch of microbiology.