An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum.〔Murray, Patrick R., and Ellen Jo. Baron. Manual of Clinical Microbiology. Vol. 1. Washington, D.C.: ASM, 2003. Print.〕〔C.Michael Hogan. 2010. ( ''Bacteria''. Encyclopedia of Earth. eds. Sidney Draggan and C.J. Cleveland, National Council for Science and the Environment, Washington DC )〕 The name "endospore" is suggestive of a spore or seed-like form (''endo'' means within), but it is not a true spore (i.e., not an offspring). It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients, and usually occurs in gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall. One side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. Revival of spores millions of years old has been claimed. When the environment becomes more favorable, the endospore can reactivate itself to the vegetative state. Most types of bacteria cannot change to the endospore form. Examples of bacteria that can form endospores include ''Bacillus'' and ''Clostridium''.
The endospore consists of the bacterium's DNA, ribosomes and large amounts of dipicolinic acid. Dipicolinic acid is a spore-specific chemical that appears to help in the ability for endospores to maintain dormancy. This chemical comprises up to 10% of the spore's dry weight.〔(【引用サイトリンク】url=http://micro.cornell.edu/research/epulopiscium/bacterial-endospores )〕
Endospores can survive without nutrients. They are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants. Thermo-resistant endospores were first hypothesized by Ferdinand Cohn after studying ''Bacillus subtilis'' (pictured to the right) growth on cheese after boiling the cheese. His notion of spores being the reproductive mechanism for the growth was a large blow to the previous suggestions of spontaneous generation. Astrophysicist Steinn Sigurdsson said "There are viable bacterial spores that have been found that are 40 million years old on Earth – and we know they're very hardened to radiation." Common anti-bacterial agents that work by destroying vegetative cell walls do not affect endospores. Endospores are commonly found in soil and water, where they may survive for long periods of time. A variety of different microorganisms form "spores" or "cysts," but the endospores of low G+C gram-positive bacteria are by far the most resistant to harsh conditions.〔(【引用サイトリンク】url=http://micro.cornell.edu/research/epulopiscium/bacterial-endospores )〕
Some classes of bacteria can turn into exospores, also known as microbial cysts, instead of endospores. Exospores and endospores are two kinds of "hibernating" or dormant stages seen in some classes of microorganisms.
Bacteria produce a single endospore internally. The spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the ''spore coat''. The spore coat, which acts like a sieve that excludes large toxic molecules like lysozyme, is resistant to many toxic molecules and may also contain enzymes that are involved in germination. The ''cortex'' lies beneath the spore coat and consists of peptidoglycan. The ''core wall'' lies beneath the cortex and surrounds the protoplast or ''core'' of the endospore. The core contains the spore chromosomal DNA which is encased in chromatin-like proteins known as SASPs (small acid-soluble spore proteins), that protect the spore DNA from UV radiation and heat. The core also contains normal cell structures, such as ribosomes and other enzymes, but is not metabolically active.
Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. Dipicolinic acid could be responsible for the heat resistance of the spore, and calcium may aid in resistance to heat and oxidizing agents. However, mutants resistant to heat but lacking dipicolinic acid have been isolated, suggesting other mechanisms contributing to heat resistance are also at work.〔Prescott, L. (1993). ''Microbiology'', Wm. C. Brown Publishers, ISBN 0-697-01372-3.〕 Small acid-soluble proteins (SASPs) are found in endospores. These proteins tightly bind and condense the DNA, and are in part responsible for resistance to UV light and DNA-damaging chemicals.〔http://micro.cornell.edu/research/epulopiscium/bacterial-endospores〕
Visualising endospores under light microscopy can be difficult due to the impermeability of the endospore wall to dyes and stains. While the rest of a bacterial cell may stain, the endospore is left colourless. To combat this, a special stain technique called a Moeller stain is used. That allows the endospore to show up as red, while the rest of the cell stains blue. Another staining technique for endospores is the Schaeffer-Fulton stain, which stains endospores green and bacterial bodies red. The arrangement of spore layers is as follows:
* Spore coat
* Spore cortex
* Core wall
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