Ultramicrobacteria

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.^[1] These cells have also been recovered from soil and appeared to be a mixture of Gram-positive and -negative species.^[2] Many, if not all, of these small bacteria are dormant forms of larger cells that allow survival under starvation conditions.^[3] 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.^[4] These starvation forms may be the most common type of ultramicrobacteria in seawater.^[5]

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.^[6] These structures are now thought to be non-living,^[7] and are probably precipitated particles of inorganic material.^[8][9]

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.^[10][11]

See also

• L-form bacteria
• Prion — smallest known infectious agent (≈10 nm)
• Parvovirus — smallest known viruses (18-28 nm)
• Nanobe — possible smallest lifeforms (20 nm)
• Nanobacteria — possible lifeforms smaller than bacteria (<200 nm)
• Mycoplasma — smallest known bacteria (300 nm)
• Nanoarchaeum — smallest known archaeum (400 nm)
• Pandoravirus — one of the largest known viruses (1000 nm)
• Pithovirus — largest known virus (1500 nm)

References

1. Torrella F, Morita RY (1 February 1981). "Microcultural Study of Bacterial Size Changes and Microcolony and Ultramicrocolony Formation by Heterotrophic Bacteria in Seawater". Appl. Environ. Microbiol. 41 (2): 518–527. PMC 243725. PMID 16345721.
2. 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.
3. 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.
4. 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.
5. Haller CM, Rölleke S, Vybiral D, Witte A, Velimirov B (February 2000). "Investigation of 0.2 µm filterable bacteria from the Western Mediterranean Sea using a molecular approach: dominance of potential starvation forms". FEMS Microbiol. Ecol. 31 (2): 153–161. doi:10.1016/s0168-6496(99)00096-3. PMID 10640668.
6. 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.
7. 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.
8. 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.
9. 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.
10. "First detailed microscopy evidence of bacteria at the lower size limit of life". ScienceDaily. Retrieved 2 March 2015.
11. Luef, Birgit; Frischkorn, Kyle R.; Wrighton, Kelly C.; Holman, Hoi-Ying N.; Birarda first5=Giovanni; Thomas, Brian C.; Singh, Andrea; Williams, Kenneth H.; Siergerist, Cristina E.; Tringe, Susannah G.; Downing, Kenneth H.; Comolli, Luis R.; Banfield, Jillian F. (27 February 2015). "Diverse uncultivated ultra-small bacterial cells in groundwater". Nature Communications 6: 6372. doi:10.1038/ncomms7372. PMID 25721682. Retrieved 2 March 2015.