An endospore is the tough, dormant protective structure that some bacteria form when conditions become harsh. It allows the organism to survive stress rather than reproduce.
Endospore formation, or sporulation, is usually triggered by nutrient shortage. The bacterium replicates its DNA, encloses one copy inside a developing forespore, and builds a resistant cortex and protein coat around it. The mature structure contains DNA, ribosomes, and protective chemicals such as calcium dipicolinate, while its dehydrated interior greatly slows chemical reactions.
Endospores can withstand heat, drying, ultraviolet radiation, freezing, and many disinfectants that would kill ordinary vegetative bacterial cells. Species of Bacillus and Clostridium are well-known endospore-formers; examples include Bacillus anthracis, Bacillus subtilis, Clostridium botulinum, and Clostridium tetani. When nutrients and other conditions improve, the endospore germinates and returns to an active bacterial cell.
The name can mislead readers: an endospore is not a reproductive “seed.” One bacterial cell generally forms one endospore, and the original mother cell is eventually destroyed. A capsule is also protective, but it is an external coating used by some active cells, not a dormant survival structure.