Azotobacter Cysts


Azotobacter is a genus of soil bacteria able to form cysts, which are dormant cells resistant to deleterious conditions. A cyst consists of a contracted oval cell, covered with a two‐layered capsule. Although many physiological and morphological studies about encystment were published decades ago, the biosynthetic pathways of the major components of the cyst (alginate, polyhydroxybutyrate and the phenolic lipids alkylresorcinols and alkylpyrones) and how they are regulated, remained largely unknown. More recent work elucidated their biosyntheses and the regulators controlling their formation and changes occurring during encystment, such as the loss of flagella. Among these regulators are the alternative sigma factors AlgU and RpoS, the global regulatory systems Gac/Rsm and the transcriptional regulators AlgR, ArpR and CydR. Lately, stress‐related proteins have been shown to contribute to the resistance of the cysts. A recent proteomic analysis confirmed many of the metabolic and structural changes observed and revealed new participants in the resistance mechanisms of the cysts.

Key Concepts

  • The encysting process results in a dormant cell more resistant to adverse conditions than the vegetative cell.
  • During the encysting process, a coordinated array of metabolic and morphological changes take place to produce a dormant cell.
  • The central body of the cyst is surrounded by a protective two‐layered capsule, which is composed of carbohydrates, proteins and lipids.
  • The exopolysaccharide alginate constitutes a structural part of the cyst envelope and is essential for desiccation resistance.
  • Several regulators of gene expression control the differentiation process, leading to the production of cyst.
  • Heat‐shock and LEA proteins prevent misfolding and aggregation of proteins exposed to stress (osmotic, freezing, heat, UV radiation, desiccation, etc.) in multiple organisms, including bacteria plants and animals.

Keywords: encystment; differentiation; dormancy; germination; capsule

Figure 1. Electron micrographs of an A. vinelandii vegetative cell (a); A vegetative cell negatively stained for visualisation of flagella (b); A cyst (c). Ex, exine; In, intine; Cb, central body; phb, polyhydroxybutyrate granules; f, flagella.
Figure 2. Diagram of the life cycle of A. vinelandii. The different development stages are depicted. The morphological changes are illustrated. The two cell types (vegetative cell and cyst) are shown with electron micrographs. The main biochemical and physiological changes occurring during the differentiation process are indicated. ARs, alkylresorcinols; APs, alkylpyrones; PHB, polyhydroxybutyrate; h, hours.
Figure 3. Model for the regulation of gene expression during encystment. Green lines: positive regulation; red lines: negative regulation. Dashed lines indicate unknown intermediates or unknown mechanism of regulation. Promoters are indicated as coloured rectangles.


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Further Reading

Berleman JE and Bauer CE (2004) Characterization of cyst cell formation in the purple photosynthetic bacterium Rhodospirillum centenum. Microbiology 150: 383–390.

Berleman JE, Hasselbring BM and Bauer CE (2004) Hypercyst mutants in Rhodospirillum centenum identify regulatory loci involved in cyst cell differentiation. Journal of Bacteriology 186: 5834–5841.

Carra S, Alberti S, Arrigo PA, et al. (2017) The growing world of small heat shock proteins: from structure to functions. Cell Stress and Chaperones 22: 601.

Castañeda M, Lopez‐Pliego L and Espín G (2016) Azotobacter vinelandii small RNAs: Their roles in the formation of cyst and other processes. In: Enguita FJ (ed.) Non‐coding RNAs and Inter‐kingdom Communication, pp 67–82. Springer International Publishing: Cham.

Dong Q and Bauer CE (2015) Transcriptome analysis of cyst formation in Rhodospirillum centenum reveals large global changes in expression during cyst development. BMC Genomics 16: 68.

Heulin T, De Luca G, Barakat M, et al. (2017) Bacterial adaptation to hot and dry deserts. In: Stan‐Lotter H and Fendrihan S (eds) Adaption of Microbial Life to Environmental Extremes, pp 75–98. Springer: Cham.

Marden JN, Dong Q, Roychowdhury S, Berleman J and Bauer CE (2011) Cyclic GMP controls Rhodospirillum centenum cyst development. Molecular Microbiology 79: 600–615.

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Segura, Daniel, Núñez, Cinthia, and Espín, Guadalupe(Jan 2020) Azotobacter Cysts. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000295.pub3]