Ultramicrobacteria (UMB) are species of the domain Bacteria, whose cells possess a volume of less than 0.1 μm3 and a small size of genome (from 3.2 to 0.58 Mb). UMB feature a combination of such determining characters as ultrasmall size of proliferating cells and small size of genome. 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 bacteria. They are characterised by a great diversity of morphology, ultrastructural organisation, physiology, biochemistry and ecology. Unique processes UMB perform are dehalorespiration, epibiont obligate and facultative parasitism, synthesis of organic compounds in oceanic waters involving bacteriorhodopsin. UMB have been found among organisms of six large phylogenetic branches of prokaryotes, where their nearest relatives are organisms with larger, typical‐of‐bacteria, cells; this is indicative of the polyphyletic origin of the currently known UMB species by reductive evolution.

Key Concepts:

  • Formation of a minimal autoreplicating 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 vistas for biotechnology.

  • Use of UMB representatives, mycoplasms, played a crucial role in epochal works by J Craig 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 are not the prohibitive factors for UMB or UMA to form.

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

Figure 1.

Electron micrographs (a, b) of an ultrathin section of nanocell aggregates as spherical structures (SS) localised as microcolonies in soil micromonoliths; (b), enlarged fragment of a peripheral part of two SS. Thin structure of SS compartments and spherical nanocells branching from SS is shown. Scale bar, 300 nm. OM, outer membrane; C, capsular fibrils; Pp, periplasm; CC, central cell in SS; CM, cytoplasmic membrane.

Figure 2.

Phase contrast micrographs of interacting cells in a binary culture of Bacillus subtilis (Bs) and Chryseobacterium solincola str. NF4 after cultivation in a starved medium for 20 min (a), 7 days (b) and 20 days (c). Ultramicrocells of strain NF4 adsorbed on Bs cells (a–c), disrupted Bs cells (b), and residual immature spores of Bs (c) are seen. BC, bacillar cell; UC, NF4 cells; Sp, spores. Scale bar, 0.5 μm.

Figure 3.

Ultramicrocells of parasitic UMB, Chryseobacterium solincola str. NF4, are adsorbed on Bacillus subtilis host cells. A contact of NF4 cell walls with the S‐layer of B. subtilis envelopes can be seen. Ultrathin section. UC, NF4 cells; N, nucleoid; Bs, B. subtilis cell; Pp, periplasm; OM, outer membrane; T, polysaccharide threads. Scale bar, 300 nm.

Figure 4.

Interaction of parasitic UMB (Kaistia adipata, str. NF1) with prey (Acidovorax delafieldii, str. 39). Fibrils of UMB capsule are attached to the host cell surface (shown by arrows). Zones of contacts of NF1 cell protrusions with the S‐layer and the cell wall of Acidovorax delafieldii 39 are visible. Ultrathin section. PF, polysaccharide fibril; SC, contact site of UMB cell protrusion with the host cell; P, prey; UC, ultramicrobacterial cell (str. NF1); Pp, periplasm; Pr, protrusion; CM, cell membrane; CW, cell wall; S, S layer. Scale bar, 0.5 μm.

Figure 5.

Enlarged fragment of an ultrathin section of interacting cells of Kaistia adipata str. NF1 and Acidovorax delafieldii str. 39 contrasted with Ruthenium Red. Threads (filaments) of UMB capsule are attached to the cell envelope of strain A. delafieldii 39; knots (globular particles on threads) can be seen. SL, tangential section of S layers; M, murein layer of cell wall; OM, outer membrane; HC, host cell (A. delafieldii); UC, 0l cell (str. NF1); T, polysaccharide threads; GP, sticky granules on threads. Scale bar, 0.3 μm. Reproduced from Duda et al..

Figure 6.

(Scheme). Microbial nanospiders attack their prey. Mechanism of cohesion of parasitic UMB (Kaistia adipata str. NF1, Chryseobacterium solincola str. NF4) and host cells (Bacillus subtilis or Acidovorax delafieldii). The following stages are shown: a B. subtilis cell captured by polysaccharide threads of strains NF4 and NF5; interacting cells approached each other under the action of Brownian motion forces; and, at the final stage, toughly adhered cell envelopes. UMB, parasite cell; HC, host cell; PF, polysaccharide fibrils; G, sticky granules on fibrils; SL, subunit layer on the surface of cell envelope. The arrows show the direction of the cohesion stages.



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Duda, Vitaly I(Nov 2011) Ultramicrobacteria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000309.pub2]