What Are Endospores?

Microorganisms are adaptive to their surroundings, they can sense and adapt accordingly to their environment. When they are in an environment which has exhausted their natural source of nutrition, they employ various survival strategies. One such strategy of bacteria is the production of endospores, usually initiated in nutrition deprivation scenarios. Endospores, as the name suggests, are seed-like formations produced within the bacteria. They are highly resistant, designed to ensure survival and preserve the genetic information under environmental stress. Endospores make survival possible for bacteria which would otherwise kill them in extreme conditions such as high temperature, pressure, chemical damage, irradiation, etc. Endospores of low gram-positive bacteria, in particular, survive through these conditions.
Also Read: Sporulation

Structure Of Endospores

Endospores have a unique cellular structure. It is surrounded by an outer covering which is proteinaceous. This coating provides enzymatic and chemical resistance to the spore. It is lined by a thick inner layer consisting of specialized peptidoglycan which forms the cortex. It is essential for the proper formation of the cortex as it brings about the dehydration of the spore, crucial to resist high temperatures. Beneath the cortex lies the cell-wall of the germ. This layer is made up of peptidoglycan which later develops to become cell wall of the bacteria post germination of endospores. Under this cell-wall, there is an inner membrane which acts as a permeability barrier protecting the bacteria against damage-causing chemicals. The core of the spore lies in the centre and is in a state of dehydration. It contains the genetic information of the bacteria such as DNA, dipicolinic acid(constitutes 10% of the dry weight of the spore), ribosomes and is crucial in the dormancy property exhibited by endospores. They also contain SASPs (Small, Acid Soluble Proteins) responsible for binding and condensing DNA. It is also responsible for the UV-light resistance property of endospores and protects them from DNA-damaging chemicals. In addition, it consists of exosporium which is an outer layer consisting of a glycoprotein.

Endospores Stain

Bacterial endospores can be visualized using a differential stain called the endospores stain. Endospores can form in the terminal, subterminal and central regions of vegetative cells. These spores are metabolically inactive and highly resistant. It produced as a defensive mechanism to survive in unfavourable conditions. A malachite green stain, which is a primary stain is applied to the endospores and the vegetative cells, which is facilitated by heat to help it penetrate through the endospores. Post this, cells are decolourized, resulting in the removal of stains from the vegetative cell only. To counterstain any cells which have been decolourized, Safranin is applied. Finally, the result is obtained with pink vegetative cells and dark green endospores. To explore more on Endospores and other related topics, register at BYJU’S.

Endospore Formation Steps

The phenomenon where an endospore is formed is known as sporogenesis or sporulation. This process is usually observed in the cells of bacteria under nutrient-deficient conditions when the endospore core tends to become increasingly dehydrated. Sporulation is a complicated process and takes seven stages to complete. Listed below are the stages:
1. Axial filament Formation – The S-I stage
The adjustment of the genetic material of the bacterial cell in the accurate centre plane
2. Septa Formation – The S-II stage
A septum known as forespore septum is formed as a result of the infolding of the plasma membrane into the cell lumen. Its creation causes a small segment of DNA to detach from the remaining genetic material.
3. Forespore Engulfment – The S-III stage
The newly formed immature spore is entirely engulfed by the membrane of the mother cell as it proceeds to grow. The engulfment causes the forespore to be enveloped by an intermembrane space and two plasma membrane.
4. Cortex Formation – The S-IV stage
The inner membrane space between the two membranes gives rise to the formation of the cortex. At this stage, the accumulation of dipicolinic acid and calcium in larger quantities is observed.
5. Protein coat Formation- The S-V stage
The cortex of the hence formed spore is covered with a protein coating
6. Spore Maturation – The S-VI stage
At this stage, the cell turns inactive metabolically and the core turns increasingly dehydrated.
7. Enzymatic destruction – The S-VII stage
Endospores are released as a result of the destruction of the enzymes of the spore mother cell. Discover furthermore about endospores and other interesting topics in biology by registering at BYJU’S.
Further Reading: Difference Between Gram-positive and Gram-negative Bacteria

Frequently Asked Questions


1. List an example of an aerobic endospore-forming bacteria.?

Bacillus subtilis


2. How can moist heat kill endospores?

It can kill endospores as the water becomes the heat that permeates in the endospore. It is possible only with moist heat and is better compared to dry heat to kill them.

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