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Ultramicrobacteria are bacteria that are smaller than 0.1 μm3 under all growth conditions.[1][2][3] This term was coined in 1981, describing cocci in seawater that were less than 0.3 μm in diameter.[4] Ultramicrobacteria have also been recovered from soil and appear to be a mixture of Gram-positive, Gram-negative
Gram-negative
and cell wall lacking species.[5][2] 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.[2] The relatively small size of ultramicrobacteria also enables parasitism of larger organisms;[2] some ultramicrobacteria have been observed to be obligate or facultative parasites of various eukaryotes and prokaryotes.[1][2] One factor allowing ultramicrobacteria to achieve their small size seems to be genome minimization[1][2] 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.[2] However, genomic data from ultramicrobacteria is lacking[2] since the study of ultramicrobacteria, like many other prokaryotes, is hindered by difficulties in cultivating them.[3] Ultramicrobacteria are commonly confused with ultramicrocells, the latter of which are the dormant, stress-resistant forms of larger cells that form under starvation conditions[1][2][6] (ie. these larger cells downregulate their metabolism, stop growing and stabilize their DNA to create ultramicrocells that remain viable for years[1][7]) whereas the small size of ultramicrobacteria is not a starvation response and is consistent even under nutrient-rich conditions.[3] The term "nanobacteria" is sometimes used synonymously with ultramicrobacteria in the scientific literature,[2] 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.[8] These structures are now thought to be non-living,[9] and likely precipitated particles of inorganic material.[10][11] See also[edit]

L-form bacteria Mycoplasma
Mycoplasma
– smallest known bacteria (300 nm) Nanoarchaeum
Nanoarchaeum
– smallest known archaeum (400 nm) Nanobacteria
Nanobacteria
– possible lifeforms smaller than bacteria (<200 nm) Nanobe – possible smallest lifeforms (20 nm) Pandoravirus – one of the largest known viruses (1000 nm) Parvovirus
Parvovirus
– smallest known viruses (18–28 nm) Pithovirus
Pithovirus
– largest known virus (1500 nm) Prion
Prion
– smallest known infectious agent (≈10 nm) ND5 and MY14ᵀ – two aerobic, Gram-negative, rod-shaped bacteria[12]

References[edit]

^ a b c d e Cavicchioli, Ricardo; Ostrowski, Martin (June 2003). Encyclopedia of Life Sciences. Nature Publishing Group. ISBN 9780470015902. Retrieved September 26, 2017.  ^ a b c d e f g h i j Duda, V; Suzina, N; Polivtseva, V; Boronin, A (2012). "Ultramicrobacteria: Formation of the Concept and Contribution of Ultramicrobacteria to Biology". Microbiology. 81 (4): 379–390. doi:10.1134/s0026261712040054.  ^ a b c Janssen, Peter; Schuhmann, Alexandra; Mörschel, Erhard; Rainey, Frederick (April 1997). "Novel anaerobic ultramicrobacteria belonging to the verrucomicrobiales lineage of bacterial descent isolated by dilution culture from anoxic rice paddy soil". Applied and Environmental Microbiology. 63 (4): 1382–1388.  access-date= requires url= (help) ^ Torrella F, Morita RY (1 February 1981). "Microcultural Study of Bacterial Size Changes and Microcolony and Ultramicrocolony Formation by Heterotrophic Bacteria
Bacteria
in Seawater". Appl. Environ. Microbiol. 41 (2): 518–527. PMC 243725 . PMID 16345721.  ^ Iizuka T, Yamanaka S, Nishiyama T, Hiraishi A (February 1998). "Isolation and phylogenetic analysis of aerobic copiotrophic ultramicrobacteria from urban soil". J. Gen. Appl. Microbiol. 44 (1): 75–84. doi:10.2323/jgam.44.75. PMID 12501296.  ^ Velimirov, B. (2001). "Nanobacteria, Ultramicrobacteria and Starvation Forms: A Search for the Smallest Metabolizing Bacterium". Microbes and Environments. 16 (2): 67–77. doi:10.1264/jsme2.2001.67. Retrieved 2008-06-23.  ^ Costerton JW, Lewandowski Z, Caldwell DE, Korber DR, Lappin-Scott HM (1995). "Microbial biofilms". Annu. Rev. Microbiol. 49: 711–45. doi:10.1146/annurev.mi.49.100195.003431. PMID 8561477.  ^ Urbano P, Urbano F (May 2007). "Nanobacteria: Facts or Fancies?". PLoS Pathog. 3 (5): e55. doi:10.1371/journal.ppat.0030055. PMC 1876495 . PMID 17530922.  ^ Kajander EO (June 2006). "Nanobacteria--propagating calcifying nanoparticles". Lett. Appl. Microbiol. 42 (6): 549–52. doi:10.1111/j.1472-765X.2006.01945.x. PMID 16706890.  ^ Raoult D, Drancourt M, Azza S, et al. (February 2008). "Nanobacteria Are Mineralo Fetuin Complexes". PLoS Pathog. 4 (2): e41. doi:10.1371/journal.ppat.0040041. PMC 2242841 . PMID 18282102.  ^ Martel J, Young JD (April 2008). "Purported nanobacteria in human blood as calcium carbonate nanoparticles". Proc. Natl. Acad. Sci. U.S.A. 105 (14): 5549–54. doi:10.1073/pnas.0711744105. PMC 2291092 . PMID 18385376.  ^ "Characterization of two aerobic ultramicrobacteria isolated from urban soil and a description of Oxalicibacterium solurbis sp. nov". FEMS Microbiology Letters. 307 (1): 25–29. 1 June 2010. doi:10.1111/j.1574-6968.2010.01954.x. 

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