Ultramicrobacteria are bacteria that are considerably smaller than typical bacterial cells and are 0.2 to 0.3 micrometres (200–300 nm) 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.
In an article published in Nature Communications in 2015, a team of researchers concentrated and cultured cells that had passed through 200-nanometer filters. The cells have an average volume of 0.009 cubic microns. Among the cells that were imaged using 2-D and 3-D cryogenic electron microscopy were some that were caught in the process of dividing. The genomes of the organisms were sequenced and found to be about one million base pairs in length.
- L-form bacteria
- Mycoplasma – smallest known bacteria (300 nm)
- Nanoarchaeum – smallest known archaeum (400 nm)
- Nanobacteria – possible lifeforms smaller than bacteria (<200 nm)
- Nanobe – possible smallest lifeforms (20 nm)
- Pandoravirus – one of the largest known viruses (1000 nm)
- Parvovirus – smallest known viruses (18–28 nm)
- Pithovirus – largest known virus (1500 nm)
- Prion – smallest known infectious agent (≈10 nm)
- ND5 and MY14ᵀ - two aerobic, Gram-negative, rod-shaped bacteria 
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