Ultramicrobacteria are bacteria that are considerably smaller than typical bacterial cells and are 0.3 to 0.2 micrometres in diameter. This term was first used in 1981, to refer to cocci in seawater that were less than 0.3 μm in diameter. These cells have also been recovered from soil and appeared to be a mixture of Gram-positive and negative species. Many, if not all, of these small bacteria are dormant forms of larger cells that allow survival under starvation conditions. In this process, cells downregulate their metabolism, stop growing and stabilize their DNA, creating dormant non-growing cells that can remain viable for many years. These starvation forms may be the most common type of ultramicrobacteria in seawater.
These small living bacterial cells are distinct from the purported "nanobacteria" or "calcifying nanoparticles", which were proposed to be living organisms that were 0.1 μm in diameter. These structures are now thought to be non-living, and are probably precipitated particles of inorganic material.
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