The domain of prokaryotic organisms, mostly single-celled. To date, about ten thousand described species of bacteria and it is assumed that they exist over a million, but the very use of the species concept to bacteria associated with a number of difficulties.
Until the end of the 1970s, the term "bacterium" was synonymous with the prokaryotes, but in 1977 on the basis of molecular biology prokaryotes have been divided into domains, archaea and eubacteria. Later, to emphasize the differences between them, they were renamed in archaea and bacteria, respectively. Although there is still a bacteria commonly understood all prokaryotes, this article describes only eubacteria. However, these two groups are similar, and many of the provisions of article are also valid for the archaea - in such cases, the term "prokaryotes", or a combination of "bacteria and archaea."
In environmental studies, and mikrobiotsenoticheskih by bacteria are often understood only nefotosinteziruyuschie nemitselialnye prokaryotes, in contrast to the functions actinomycetes and cyanobacteria.