Endospore
From Palaeos
(1) The nearly immortal resting state of some bacteria. Endospores are composed of a central spore cell, which is surrounded by various protective layers. The outermost layer is the exosporium, which is a thin covering made of protein. Below this is the spore coat which is made up of highly cross-linked keratin and layers of spore-specific proteins. The cortex consists of loosely cross-linked peptidoglycan. The innermost spore cell contains the components of the vegetative bacterial cell (the cell wall, cytoplasmic membrane, cytoplasm, nucleoid, etc.). ... The water content of endospores is only about 10-30% of the water content of vegetative cells; therefore, endospores are capable of surviving at levels of dehydration that would kill vegetative cells. The low water content also provides the endospore with chemical resistance (to chemicals such as hydrogen peroxide) and it causes the remaining enzymes of the spore cell to become inactive. One chemical produced by endospores that is thought to lend to their high resistance is dipicolinic acid. This chemical has been found in the spore cell of all endospores examined. Dipicolinic acid interacts with calcium ions to form calcium dipicolinate, which is the main substance believed to lend endospores their resistance and represents about 10% of the dry weight of an endospore. The spore cell also contains small acid-soluble spore proteins (SASPs). These function to protect DNA from UV radiation, dessication and dry heat, and they also serve as a carbon and energy source during the germination process (conversion back to a vegetative cell). Another component of endospores that contributes to their resistant to chemical agents is the strong spore coat, which is composed of highly cross-linked keratin. Identification of particular organisms can be aided by the presence, location and size of endospores. Endospores can be located centrally, terminally or subterminally within a cell. Sometimes the endospore is much larger in diameter than the cell, which causes the cell to appear swollen at the location of the endospore.
(2) In the Microsporidia, a protective coat of chitin located between the outer protein exospore and the cell membrane.
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