WebBacteria (/ b æ k ˈ t ɪər i ə / ⓘ; sg.: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell.They constitute a large domain of prokaryotic microorganisms.Typically a few micrometres in length, 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 ...