Ultramicrobacteria are species of the domain Bacteria, whose cells possess a volume of less than 0.1 μm3 and a small size of genome. Ultramicrobacteria are bacterias that are smaller than typical bacterial cells and so are substantially 0.3 to 0.2 micrometres in size. This term was initially found in 1981, to make reference to cocci in seawater which were significantly less than 0.3 μm in size. These cells are also recovered from soil and were an assortment of Gram-positive and -detrimental species.