An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seedlike form (endo means within), but it is not a true spore (i.e. not an offspring). It is a stripped-down, dormant form that the bacterium can reduce itself to. The endospore becomes important when the bacterium is experiencing an environment that is deleterious to the usual vegetative state of the bacterium, such as in desiccating conditions. Endospores enable bacterium to survive periods of environmental stress lasting at least several thousand years, and revival of spores many 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, but examples include Bacillus and Clostridium.
The endospore consists of the bacterium's DNA and part of its cytoplasm, surrounded by a very tough outer coating.
Endospores can survive without nutrients. They are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants. Common anti-bacterial agents that work by destroying vegetative cell walls don't work on endospores. Endospores are commonly found in soil and water, where they may survive for long periods of time.
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.
In contrast to eukaryotic spores, which are produced by many eukaryotes for reproductive purposes, bacteria will 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 is encased in chromatin-like proteins known as SASPs, 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.
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