Bacteria ( ; sg.: #bacterium) are ubiquitous, mostly free-living organisms often consisting of one #biologicalCell. 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 the air, soil, water, #acidicHotSprings, #radioactiveWaste, and the #deepBiosphere of #EarthsCrust.