Ultramicrobacteria

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][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 non-coding DNA, 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]

Microbacterial studies from Berkeley Labs at UC Berkeley have produced detailed microscopy images of ultra-small microbial species.[6] 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.[6] 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,[6] display dense spirals of DNA, few ribosomes, hair-like fibrous appendages, and minimized metabolic systems.[6] Such cells probably gain most essential nutrients and metabolites from other bacteria.[6] 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[1][2][7] (i.e. these larger cells downregulate their metabolism, stop growing and stabilize their DNA to create ultramicrocells that remain viable for years[1][8]) 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.[9] These structures are now thought to be nonliving,[10] and likely precipitated particles of inorganic material.[11][12]