Ultramicrobacteria Explained

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 and cell-wall-lacking species.^[5] 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 non-coding DNA, 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.

Microbacterial studies from Berkeley Labs at UC Berkeley have produced detailed microscopy images of ultra-small microbial species. Cells imaged have an average volume of 0.009 μm³, meaning that about 150,000 of them could fit on the tip of a human hair. These bacteria were found in groundwater samples and analyzed with 2-D and 3-D cryogenic transmission electron microscopy. These ultra-small bacteria, about 1 million base pairs long, display dense spirals of DNA, few ribosomes, hair-like fibrous appendages, and minimized metabolic systems. Such cells probably gain most essential nutrients and metabolites from other bacteria. Bacteria in the ultra-small size range are thought to be rather common but difficult to detect. ^[6]

Ultramicrobacteria are commonly confused with ultramicrocells, the latter of which are the dormant, stress-resistant forms of larger cells that form under starvation conditions^[7] (i.e. these larger cells downregulate their metabolism, stop growing and stabilize their DNA to create ultramicrocells that remain viable for years^[8]) 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.^[9] These structures are now thought to be nonliving,^[10] and likely precipitated particles of inorganic material.^{[11] [12]}

See also

• 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)
• Pithovirus – largest known virus (1,500 nm)
• Pandoravirus – one of the largest known viruses (1,000 nm)
• Parvovirus – smallest known viruses (18–28 nm)
• Prion – smallest known infectious agent (≈10 nm)
• ND5 and MY14^T – two aerobic, Gram-negative, rod-shaped bacteria^[13]

Notes and References

1. Book: Cavicchioli. Ricardo. Ostrowski. Martin. Encyclopedia of Life Sciences. June 2003. Nature Publishing Group. 9780470015902. September 26, 2017.
2. Duda. V. Suzina. N. Polivtseva. V. Boronin. A. Ultramicrobacteria: Formation of the Concept and Contribution of Ultramicrobacteria to Biology. Microbiology. 2012. 81. 4. 379–390. 10.1134/s0026261712040054. 23156684. 6391715.
3. Janssen. Peter. Schuhmann. Alexandra. Mörschel. Erhard. Rainey. Frederick. Novel anaerobic ultramicrobacteria belonging to the verrucomicrobiales lineage of bacterial descent isolated by dilution culture from anoxic rice paddy soil. Applied and Environmental Microbiology. April 1997. 63. 4. 1382–1388. 10.1128/AEM.63.4.1382-1388.1997. 9097435. 168432. 1997ApEnM..63.1382J.
4. Torrella F, Morita RY . Microcultural Study of Bacterial Size Changes and Microcolony and Ultramicrocolony Formation by Heterotrophic Bacteria in Seawater . Appl. Environ. Microbiol. . 41 . 2 . 518–527 . 1 February 1981. 10.1128/AEM.41.2.518-527.1981 . 16345721 . 243725 . 1981ApEnM..41..518T .
5. Iizuka T, Yamanaka S, Nishiyama T, Hiraishi A . Isolation and phylogenetic analysis of aerobic copiotrophic ultramicrobacteria from urban soil . J. Gen. Appl. Microbiol. . 44 . 1 . 75–84 . February 1998 . 12501296 . 10.2323/jgam.44.75. free .
6. Web site: Krotz, D. . First Detailed Microscopy Evidence of Bacteria at the Lower Size Limit of Life . 2015 . 2020-05-11.
7. Velimirov, B. . 2001 . Nanobacteria, Ultramicrobacteria and Starvation Forms: A Search for the Smallest Metabolizing Bacterium . Microbes and Environments . 16 . 2 . 67–77 . 10.1264/jsme2.2001.67 . free .
8. 1995. Microbial biofilms. Annu. Rev. Microbiol.. 49. 711–45. 10.1146/annurev.mi.49.100195.003431. 8561477. Costerton JW, Lewandowski Z, Caldwell DE, Korber DR, Lappin-Scott HM.
9. Urbano P, Urbano F . Nanobacteria: Facts or Fancies? . PLOS Pathog. . 3 . 5 . e55 . May 2007 . 17530922 . 1876495 . 10.1371/journal.ppat.0030055 . free .
10. Kajander EO . Nanobacteria--propagating calcifying nanoparticles . Lett. Appl. Microbiol. . 42 . 6 . 549–52 . June 2006 . 16706890 . 10.1111/j.1472-765X.2006.01945.x. 20169194 .
11. Raoult D, Drancourt M, Azza S, etal . Nanobacteria Are Mineralo Fetuin Complexes . PLOS Pathog. . 4 . 2 . e41 . February 2008 . 18282102 . 2242841 . 10.1371/journal.ppat.0040041 . free .
12. Martel J, Young JD . Purported nanobacteria in human blood as calcium carbonate nanoparticles . Proc. Natl. Acad. Sci. U.S.A. . 105 . 14 . 5549–54 . April 2008 . 18385376 . 10.1073/pnas.0711744105 . 2291092. free .
13. Characterization of two aerobic ultramicrobacteria isolated from urban soil and a description of Oxalicibacterium solurbis sp. nov.. 1 June 2010. FEMS Microbiology Letters. 307. 1. 25–29. 10.1111/j.1574-6968.2010.01954.x. 20370834. Sahin. Nurettin. Gonzalez. Juan M.. Iizuka. Takashi. Hill. Janet E.. free.