Ultramicrobacteria (UMB) are characterised by the following properties:

  1. UMB are species of prokaryotic organisms belonging to the domain Bacteria.
  2. UMB cells possess extremely small size; their volume is ≤0.1 μm3 irrespective of growth conditions and development cycle.
  3. UMB feature a very small size of genome (mainly from 3.2 to 0.58 Mb).
  4. An obligate capability of self‐reproduction.

UMB feature a combination of such determining characters as ultrasmall size of proliferating cells, small size of genome and capability of self‐reproduction. As a synonym for UMB, some authors use the term ‘nanobacteria’ (NB).

Several dozens of UMB species have been isolated from various habitats, such as aquatic and soil environments, sediments, silts, Greenland ice, permafrost, human intestines and insects; and are cultured under laboratory conditions on various nutritive media. The term ‘nan(n)obacteria’ is also used to designate ultrasmall bacterium‐like particles occurring in rocks, sands, soils, in deep subsurface, meteorite and clinical samples. UMB include species of free‐living and parasitic (predatory) bacteria. Their cells divide by constriction, septation or budding. Unique processes UMB perform are dehalorespiration and epibiont obligate and facultative parasitism. UMB have been found among organisms of six large phylogenetic branches of prokaryotes.

Key Concepts

  • Formation of a minimal self‐reproducing microbial cell is the prerogative of the prokaryotes.
  • Ultrasmall size of cells enables parasitism (predation) of UMB on ‘large‐cell’ species of prokaryotes.
  • UMB are important model objects for studies of genome evolution in prokaryotes.
  • UMB studies are of vital importance for solving issues of the origin and evolution of primeval living objects.
  • UMB research opens new trends in biotechnology.
  • Use of UMB representatives, mycoplasms, played a crucial role in pioneering works by J. C. Venter and collaborators for experimentally creating a new living being (cells with chemically synthesised genome) and a new bacterial species.
  • Description of new UMB cell structures expands the diversity pattern of the ultrastructural and molecular organisation of the prokaryotic cell.
  • Gram‐negative, Gram‐positive or archaeal type of cell organisation is not the prohibitive factors for UMB or ultramicroarchaea to form.

Keywords: ultramicrobacteria (UMB); nanobacteria/nannobacteria (NB); ultramicrocells (UmC); nanocells (NC); natural minimal cells; intermicrobial parasitism; reductive evolution

Figure 1. Ultramicrobacteria in a microbial fraction separated from light chestnut soil (Rostov Region, Russia). Some smallest cells are seen; their diameter, about 0.2 μm. Green cells, live; red cells, dead. Viewed in a luminescent microscope after Live/Dead cell staining. Scale bar, 10 μm.


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Duda, Vitaly I, Suzina, Natalia E, and Boronin, Alexander M(Mar 2020) Ultramicrobacteria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000309.pub3]