Structure of Endospore:
Endospores are formed only in a few bacteria in adverse conditions. The endospores are known as resting spores and represent the resting stages of bacteria. The protoplast becomes rounded and secretes a hard, resistant wall around it. Each cell forms a single spore known as an endospore. They are very resistant to temperatures as low as ice and as high as boiling water. They withstand dry conditions and remain alive in poisonous chemical substances.
During unfavorable conditions, they remain dominant for a long time and are set free from the parent cell when the conditions escape as bacterial cells. Sometimes more than one endospore may be formed in the parent cell as in Metabacterium polypore.
Endospores are bodies produced inside the cells of a considerable number of bacterial species. They are more resistant to unfavorable environmental conditions, such as heat, cold, desiccation, osmosis, and chemicals than the vegetative cell producing them. The bulk of evidence indicates the existence of a close relationship between spore formation and the exhaustion of nutrients essential for vegetative growth.
Sporulation is a defense mechanism to protect the cell when the occasion arises. Spore formation is limited to two genera Bacillus and Clostridium. The spore formation is much more complex than that of vegetative cells in that it has many layers. The outermost is the exosporium, a thin delicate covering, and within this is the spore coat, which is composed of a layer or layers of wall-like material. Below the spore coat is the cortex, a region consisting of many concentric rings like the cell wall, the cortex is composed of peptidoglycan. Inside the cortex are the usual cell wall, cell membrane, nuclear region, and other cell components.