<P> An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum . 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

What kinds of conditions is an endospore resistant to
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