When bacteria belonging to the genus Bacillus or Clostridium run out of essential nutrients, they produce endospores. Endospores allow the bacteria to survive extreme conditions that are not optimal for growth. If conditions become favorable, the endospore can go through a process called germination and form a new vegetative cell. Endospores are dehydrated structures and are not actively metabolizing. Endospores are also resistant to heat, radiation, acids, and many chemicals such as disinfectants that normally harm and kill vegetative cells. Endospores are resistant due to their protein coat or exosporium that forms a protective barrier around the spore. Heat resistance is directly correlated with water content. The higher the water content of the cell the less resistant to heat it will be. The water content of the spore is 10-30%. The water content is low because calcium ions form a complex with proteins and a dipicolinic acid. The complex forms a gel that controls how much water can enter the cell. The only way to destroy endospores is to expose them to steam under pressure which generates temperatures of 121 degrees celsius. An autoclave can create such conditions.
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