Overview of Endospore

• Endospores are dormant, tough, resistant, and non-reproductive structures which are produced by gram-positive bacterial cell (such as bacillus, clostridium) in the phylum firmicutes during the unfavorable condition of the environment.
• It developed within the vegetative cells and help them to endure the unfavorable environmental conditions.
• Endospore is resistant to different environmental stress such as heat, UV, gamma radiation, chemical disinfectants, and desiccation.
• They can remain viable for around 100,000 years.
• The lack of nutrient trigger the endospore formation process.
• There are several endospore-forming bacteria that are highly pathogenic in nature such as Clostridium botulinum, Bacillus anthracis, C. tetani (causes tetanus) and C. perfringens (causes gas gangrene and food poisoning).
• Alcohol or hydrogen peroxide or boiling at 100-degree centigrade will not kill the bacterial endospore. But autoclaving at 121 degree centigrade can kill the endospore.
• The mother cell that produces endospore is known as sporangium.
• A specilaized stain called malachite green is used to visulize endospore under microscope. 

Structure of Endospore

1. The structure of the endospore is observed under an electron microscope. The electron micrograph shows that, endospore consists of a core surrounded by different layers.
2. The composition of core is similar to the composition of ribosome and nucleoid but the water content is very low.
3. The core is surrounded by a layer called the inner membrane.
4. The inner membrane is surrounded by the core wall or germ cell wall. The germ cell wall is made up of peptidoglycan.
5. The surrounding region of the core wall is known as the cortex which is made up of peptidoglycan. The cortex occupy the half volume of the spore.
6. A phospholipid bilayer is surrounded the cortex, which is known as outer membrane.
7. The outer membrane is surrounded by a region called coat. The coat is made of 50 different proteins which are highly crossed linked.
8. A thin and delicate layer covered the spore, which is known as endosporium.

Location of Endospore

Based on the location of endospore within the bacterial cell they are classified into different groups such as;

1. Central: When the endospore is located at the center of bacterial cell.
2. Swollen Sporangium: when the mature endospore is located in a characteristic location in the mother cell is referred to as the sporangium.
3. Subterminal: When the endospore is  located close to an end of bacterial cell is known as subterminal.
4. Terminal: When the endospore is located at the end of a bacterial cell is known as trminal.

Resistance of Endospore

The resistance property of endospore is dependent on different factors such as;

1. All those layers also responsible for the resistance property of the endospore. The  spore coat protects prevents different chemicals and lytic enzymes such as lysozyme.
2. The inner membrane is impermeable to different chemicals that are cause DNA damage.
3. The low water content, lower pH, and the presence of a high amount of DPA and the formation of Ca-DPA (calcium and dipicolonic aicd) complex in the spore core is responsible for the resistance property.
4. The spore DNA also protected by two different mechanisms such as;
   1. The Ca-DPA complex inserted into the nitrogen bases of DNA and stabilize them.
   2. A small, acid-soluble DNA binding protein (SASPs) saturate the spore DNA, and make them stable.
5. The high concentration of calcium ion prevents the damages causes by wet heat and oxidizing agents.
6. The endospore contain different types of DNA repair enzymes which quickly heal the damaged DNA when the cell are exposed in harshed environmental condition.
7. The cortex region removed water from the core osmotically (causes dehydration). This dehydration condition provides resistance to the endospore.
8. SASPs bind with the spore DNA and cause conformational changes in DNA, which lead to the formation A form of DNA from B DNA. The A DNA is more compact as compared to B DNA. A DNA prevents the formation of pyrimidine dimer by UV radiation.

Endospore Formation Steps

The processes by which endospore is formed known as sporulation. The sporulation occurs when the cell suffers lack of nutrition. The sporulation processes is completed in seven steps such as;

1. Formation of axial filaments, In this step the spore DNA is replicated and the genetic material is oriented in the exact central plane of the bacterial cell.
2. The formation of forespore septum, It involves the inward growth of bacterial cell membrane and as a result, it develops a forespore septum. During the formation of the septum, it separates a small portion of DNA from the rest of the genetic materials.
3. Engulfment of Septum, In this step the cell membrane of the mother cell grows continuously, thus it engulf the newly formed septum or immature spore. Now the cell contains two cell membranes and an intermembrane space.
4. Formation of cortex, the cortex region is developed between the two cell membrane or in the intermembrane space. The DPA and calcium ion also started to accumulate.
5. Formation of coat, a protein coat is started to laid down over the cortex region.
6. Maturation of Endospore, In this step the coat formation is completed, spore increases the refactility and heat resistance. The spore becomes metabolically inactive.
7. Enzymatic destruction, In this step different lytic enzymes are released which lysis the sporangium (Mother cell of endospore) and release the endospore.

Formation of Vegetative cell from endospore or Endospore Germination

The formation of vegetative cells from endospore is triggered when the environmental condition becomes favorable. In this step involves three distinct steps such as;

1. Activation: In this step, the endospore is getting prepared for the germination. This step is initiated when the endospore is heated.
2. Germination: In this step involve the breaking of the spore’s dormant state. This occurs when the germinate acceptor on the inner membrane detects small molecules such as sugar and amino acids. After detection of different small molecules, it initiates a series of events such as;
   1. Swelling of endospore
   2. Release of Ca-DPA complex.
   3. Activation of different enzymes.
   4. Uptake of water.
   5. Rapture of spore cortex regions.
   6. Loss of refactility.
3. Overgrowth: In this step, the inactive endospore is converted into a metabolically active vegetative cell. Different types of nutrients are started to synthesis for the overgrowth of the endospore.

Endospore Definition, Formation, Germination, Structure, Resistance Video