Endospore

Endospore

​Certain Gram(+)ve bacteria produce special structures that are able to overcome unfavourable conditions such as heat, UV radiation, nutrient deficiency conditions. This structure is called endospore.

​Endospore produced from a vegetative cell is called sporulation.

• ​Example: Bacillus, Clostridium, Bacillus anthracis, Clostridium botulinum.

​Endospores are produced endogenously within the cell. The structure of the spore, as seen with the electron microscope, is vastly different from that of a vegetative cell. It has many layers:

1. ​Exosporium: A thin delicate covering made up of protein.
2. ​Spore coat: Within the exosporium, there is a spore coat, composed of layers of spore-specific protein.
3. ​Cortex: Below the spore coat is the cortex, which consists of loosely arranged crosslinked peptidoglycan.
4. ​Core: Inside the cortex, the core or core protoplast contains the usual cell wall and cytoplasmic membrane.

​One chemical substance characteristic of endospores but not present in vegetative cells is dipicolinic acid.

​Spore Formation

​Spore formation, sporogenesis, or sporulation normally commences when growth ceases due to lack of nutrients. It is a complex process and may be divided into 7 stages:

1. ​An axial filament of nucleic material forms.
2. ​Followed by an inward folding of the cell membrane to enclose part of the DNA and produce the forespore septum.
3. ​The membrane continues to grow and engulfs the immature spore in a second membrane.
4. ​Cortex is laid down in the space between the two membranes, and both calcium and dipicolinic acid are accumulated.
5. ​Protein coats are then formed around the cortex.
6. ​Maturation of spore occurs.
7. ​Finally, lytic enzymes destroy the sporangium, releasing the spore.

​Sporulation requires only about 10 hours.

​Why endospores are called heat resistant?

​Ans:

1. ​Having an abundant calcium dipicolinate content, the core is in a partially dehydrated state. The core of a mature endospore contains only 10 to 30% of the water content of the vegetative cell. The core thus greatly increases the heat resistance of the spore and is also resistant to chemicals like H₂O₂.
2. ​In addition to low water content, the pH of the core cytoplasm is about one unit lower than the vegetative cell.
3. ​The core contains high levels of core-specific proteins called Small Acid-Soluble Spore Proteins (SASPs), which bind tightly to the DNA in the core and protect it from UV radiation. Besides, SASPs function as a carbon and energy source for the outgrowth of a new vegetative cell from the endospore.