Giant Bacteria


For almost all bacteria, it is convenient to be very small, but a few highly specialist groups of bacteria have evolved to be orders of magnitude larger than ordinary bacteria. While some of these giant microbes are abnormally large in all dimensions, others are long and thin or consist of assemblages of multiple cells. These giant bacteria are spread across the domain Bacteria and have evolved multiple ways to combat diffusion constrains. Giant bacteria often thrive in nutrient and carbon‐rich environments and/or in steep redox gradients. Genomically, giant bacteria seem all to be polyploid, that is containing multiple copies of their genome. Giant bacteria have been described throughout the history of microbial research but only some of them can be traced in current taxonomy and are being actively studied. Therefore, it is likely there are other giant bacteria in nature, awaiting to be explored.

Key Concepts

  • Growth and activity of giant bacteria are expected to be constrained by a combination of diffusion of nutrients into the cells as well as intracellular trafficking of solutes by diffusion.
  • Giant bacteria can be divided into ‘truly’ giant, consisting of abnormally large cells on all dimensions, or ‘pseudo‐giant’ consisting of cells abnormally large on one axis or multicellular assemblages of bacteria.
  • ‘Pseudo‐giant’ bacteria may not face the same diffusion limitations as ‘truly’ giant bacteria.
  • ‘Truly’ giant bacteria likely ‘resolve’ diffusion constrains by one or more methods including, minimising cytoplasmic space by intracellular (pseudo)compartmentalisation, storage vacuoles, rapid swimming.
  • Giant bacteria often grow in carbon‐ and nutrient‐rich environments and/or in steep redox gradients, thus additionally minimising diffusion‐related constraints.
  • Current knowledge suggests that all giant bacteria contain multiple copies of their genomes being oligoploid (3–10 copies) or polyploid (10–1000s of copies).
  • Gigantism in bacteria occurs in multiple phylogenetic lineages covering both Gram‐negative and Gram‐positive taxa.
  • Early descriptions of giant bacteria suggest that the currently studied taxa do not encompass the entire diversity of abnormally large bacteria.

Keywords: Spirochaetes; Epulopiscium; Cyanobacteria; sulfur bacteria; cable bacteria; diffusion; polyploidy; ‘truly’ giant bacteria; pseudo‐giant bacteria

Figure 1. A bundle of Thioploca filaments from marine sediments close to the Peruvian coast. The bundle is placed next to a ruler of 5 cm in length. The Thioploca filaments appear white due to internal sulfur granules that reflect the light.
Figure 2. Large intestinal symbionts of surgeonfish. These panels, all shown at the same magnification, illustrate the morphological diversity of ‘Epulopiscium’ and its relatives. The cell on the left is the second largest morphotype, Epulopiscium type B. Visible are two large internal offspring cells. Other rod‐shaped and filamentous forms are shown. Scale bar represents 20 μm (Angert, ). The enigmatic cytoarchitecture of Epulopiscium spp. Reproduced with permission from Fuerst . © Springer Nature.
Figure 3. Examples of large‐celled Cyanobacteria. All scale bars are 10 μm. (a) Chroococcus sp., dividing (left) and recently divided cells from this typically freshwater genus. (b) Porphirosiphon notarissi, a filamentous cyanobacterium allied to the large Oscillatoria, isolated from a soil crust in the Namibian desert, displaying the typical disc‐shaped cells in two separate trichomes, enclosed in a common extracellular sheath. Note the apoptotic, vestigial cell between the two trichomes that served to separate them in a process of self‐immolation. (c) Detail of a morphologically complex Stigonema spp. displaying a main filament consisting of two series of cells, and an incipient uniseriate branch.
Figure 4. The morphology of the giant sulfur bacteria as they appear in the light microscope: (a) Beggiatoa, (b) Thioploca, (c) Thiothrix, (d) Thiomargarita, (e) Thiovulum, (f) Achromatium. In (a–e), numerous sulfur inclusions can be seen. Achromatium (f) is the only known bacterium with calcite inclusions. The scale bars show typical sizes for each genus. Nevertheless, the cell diameters of sulfur bacteria are highly variable.


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Ionescu, Danny, and Bizic, Mina(Jul 2019) Giant Bacteria. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0020371.pub2]