Ultramicrobacteria are bacteria that are smaller than 0.1 μm3 under
all growth conditions. This term was coined in 1981,
describing cocci in seawater that were less than 0.3 μm in
Ultramicrobacteria have also been recovered from soil and
appear to be a mixture of Gram-positive,
Gram-negative and cell wall
Ultramicrobacteria possess a relatively high
surface-area-to-volume ratio due to their small size, which aids in
growth under oligotrophic (i.e. nutrient-poor) conditions. The
relatively small size of ultramicrobacteria also enables parasitism of
larger organisms; some ultramicrobacteria have been observed to be
obligate or facultative parasites of various eukaryotes and
prokaryotes. One factor allowing ultramicrobacteria to achieve
their small size seems to be genome minimization such as in the
case of the ultramicrobacterium P. ubique whose small 1.3 Mb genome is
seemingly devoid of extraneous genetic elements like nonworking genes,
transposons, extrachromosomal elements etc. However, genomic data
from ultramicrobacteria is lacking since the study of
ultramicrobacteria, like many other prokaryotes, is hindered by
difficulties in cultivating them.
Ultramicrobacteria are commonly confused with ultramicrocells, the
latter of which are the dormant, stress-resistant forms of larger
cells that form under starvation conditions (ie. these larger
cells downregulate their metabolism, stop growing and stabilize their
DNA to create ultramicrocells that remain viable for years)
whereas the small size of ultramicrobacteria is not a starvation
response and is consistent even under nutrient-rich conditions.
The term "nanobacteria" is sometimes used synonymously with
ultramicrobacteria in the scientific literature, but
ultramicrobacteria 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 likely precipitated particles of inorganic
Mycoplasma – smallest known bacteria (300 nm)
Nanoarchaeum – smallest known archaeum (400 nm)
Nanobacteria – possible lifeforms smaller than bacteria
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
^ a b c d e Cavicchioli, Ricardo; Ostrowski, Martin (June 2003).
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^ Torrella F, Morita RY (1 February 1981). "Microcultural Study of
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^ Velimirov, B. (2001). "Nanobacteria,
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^ Raoult D, Drancourt M, Azza S, et al. (February 2008). "Nanobacteria
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