[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.
[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.
[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.
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] 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.