Azotobacter Cysts

Abstract

Azotobacter is a genus of Gram‐negative, organotrophic, nitrogen‐fixing soil bacteria. These bacteria undergo a differentiation process to form cysts, which are dormant cells resistant to deleterious conditions. A cyst consists of a contracted oval cell, called central body, covered with a two‐layer capsule. Although the process of encystment has been known for many years and many physiological and morphological studies were published decades ago, the biosynthetic pathways of the major components of the cyst and how they are regulated remained largely unknown. More recent work has elucidated the biosynthesis and genetics of some of these components, like alginate, polyhydroxybutyrate and the phenolic lipids alkylresorcinols and alkylpyrones and also the regulators controlling their biosynthesis and the 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.

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 production of cyst.

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.

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

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Suzina NE, Mulyukin AL, Dmitriev VV et al. (2006) The structural bases of long‐term anabiosis in non‐spore‐forming bacteria. Advances in Space Research 38: 1209–1219.

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Segura, Daniel, Núñez, Cinthia, and Espín, Guadalupe(Feb 2014) Azotobacter Cysts. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000295.pub2]