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 diameter. Ultramicrobacteria have also been recovered from soil and appear to be a mixture of Gram-positive, Gram-negative and cell-wall-lacking species. Ultramicrobacteria possess a relatively high surface-area-to-volume ratio due to their small size, which aids in growth under oligotrophic 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. Recent 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 cubic microns, meaning that about 150,000 of them could fit on the tip of a human hair. These bacteria were found in groundwater samples from Rifle, Colorado, after the water was filtered through filters commonly believed to sterilize water. These samples were then brought to Berekeley's Lab's Life Sciences Division, and analyzed with 2-D and 3-Dcryogenic transmission electron microscopy. Images of theses ultra-small bacteria 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. Genomic sequencing of specimens found the genomes to be about 1 million base pairs long, and that the species found came from multiple phyla. Bacteria in the ultra-small size range are thought to be rather common but difficult to detect. Ultramicrobacteria are commonly confused with ultramicrocells, the latter of which are the dormant, stress-resistant forms of larger cells that form under starvation conditions 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 material.
