Endospore
An endospore is a structure made by bacteria for survival purposes. Bacteria turn to endosporea when they undergo stress, such as an increase in heat or lack of nutrients. This process is known as sporulation. Endospores are small, rounded, resting cells that form inside bacteria when conditions become unbearable. They have a tough coating which makes it resistant to radiation (like X-rays and UV light), chemicals, heat, salt concentration and extreme pH. Endospores have calcium dipicolinate in them, which keeps the DNA stable.  It is not fully understood what causes it to be so resistant.