Certain bacteria can form endospores in dry environments in a process called sporulation. They are called endospores because the spores form within the cell. Endospores offer great advantages to bacterial cells because they are extremely resistant to a number of harsh environments, including: heat, desiccation, radiation, chemicals, acids, and drying. The discovery of bacterial endospores was important to microbiology because it provided knowledge on proper methods for sterilization of culture media, foods, and other perishable items. Many organisms form spores, but the bacterial endospore is unique in its heat resistance capabilities. Yet, how are endospores so resistant to harsh environments? The answer lies in the structure of the endospore illustrated below:

"Endospore Structure" (3)

Spore Structure:

Exosporium - A thin delicate covering made of protein.
Spore coats - Composed of layers of spore specific proteins.
Cortex - Composed of loosely linked peptidoglycan and contains dipicolinic acid (DPA), which is particular to all bacterial endospores. The DPA cross links with calcium ions embedded in the spore coat. This cross linkage greatly contributes to the extreme resistance capabilities of the endospores because it creates a highly impenetrable barrier. The calcium DPA cross linkages compose 10% of the dry weight of the endospores.

"Calcium Endospore Cross Linkages Confer Resistance" (4)

Core - The core contains the usual cell wall and, cytoplasmic membrane, nucleoid, and cytoplasm. The core only has 10-30% of the water content of vegetative cells; therefore the core cytoplasm is in a gel state. The low water content contributes to the endospores success in dry environments. However, the low water concentration and gel cytoplasm contributes to the inactivity of cytoplasmic enzymes. The core cytoplasm is also one unit lower in pH than the vegetative cell, thus conferring acidic environment survival. SASPs, small acid soluble spore proteins, are formed during sporulation and bind to DNA in the core. SASPs protect the DNA from UV light, desiccation, and dry heat. SASPs also serve as a carbon energy source during germination, the process of converting a spore back to a vegetative cell.