Spores are the dormant structures of prokaryotes. Endospores are the differentiated cells that are especially resistant to various environmental stresses and remain dormant. The bacterial spores develop within bacterial cell, so they are called endospores.
Bacterial genera that form endospores
Endospore is formed characteristically in gram-positive bacteria only. The genera that possess capability to form spores include-- Bacillus
- Clostridium
- Sporolactobacillus
- Sporosarcina
- Desulfatomaculum
- Paeniabacdllus (Earlier this was member of genus Bacillus)
- Thermoactinomyces (This actinomyces are endo-spore formers)
Properties of endospores :
- Bacterial spores are the dormant structures, which can perform up to 100000 years. They do not perform any metabolic activity. Such a state of dormancy is known as Cryptobiosis.
- Bacterial spores are extraordinarily resistant to environmental stresses like heat, radiations (ultraviolet rays. gamma rays), various chemical disinfectants, and desiccation.
- Spores are highly resistant to staining process. This is mainly due to the impermeability of spore wall.
- They are the refractile bodies, which cannot be stained easily by usual methods of staining.
Shapes and location of spore :
Bacterial spores are formed with in the cell. Hence, they are called endospores. Their shape and position within the cell is characteristic of the species by which It is formed
- They can be central, sub terminal or terminal in their position within the cell.
- Usually their size is same as that of cell size. But some bacteria produce spores having diameter greater than that of cell.
- They may be round or oval in shape. having smooth or wrinkled surface.
Structure of endospore :
Detailed ultra structure of the endospore can be studied by transmission electron microscopy. The endospore has a very complex structure, consisting of -
- Exosporium
- Spore coat
- Cortex
- Spore protoplast
Exosporium
- Exosporium is a thin and delicate covering of spore. It is supposed to be originated from cell membrane during sporulation.
Spore coat
- A layer located below the exosporium is called spore coat. It is thick and consists of several layers of proteins.
- It is impermeable to toxic chemicals and thus account for resistance to chemicals.
- It is believed to contain enzymes for germination.
Cortex
- Cortex is a structure below the spore coat. It forms more than half of the spore volume.
- It contains relatively less cross-linked peptidoglycans.
- It also possesses a high content of calcium dipicolonate (Ca-DPA).
- They constitute about 15% of the cellular dry weight.
Spore protoplast
- It is the core region of spore inside the cortex. It consists of core wall (spore wall), the cytoplasmic membrane, cytoplasm, ribosomes and nucleoid.
- It is metabolically Inert.
- The water content in spore protoplast exists as bound water. water complexed with protein.
Factors associated with resistance in endospore
Heat resistance of bacterial spores is attributed to various factors. They Include- Thick layer of spore coat and cortex.
- High content of Ca-DPA present in cortex of spore
- Presence of very little amount of free water in spore protoplast.
Endospore formation- Sporulation or sporogenesis
- Sporogenesis is a complex process of differentiation that in several stages.
- Sporulation results differentiation of vegetative bacterial cell in to altogether a different morphological structure, recognised as spore.
- The process of sporogenesis is a genetically defined character and it involves about 200 genes. The proteins produced by these genes catalyse series of events concerned with sporogenesis.
- The process of sporulation takes about ten hours.
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| Stages of endospore formation |
Stage 1.
- It Involves coalescence of replicated nucleoids. The a compact and elongate nuclear material forms structure which gets arranged as axial filament.
Stage 2.
- Now, the cell membrane invaginates or folds inward to form a septum. It divides the cell unequally, Smaller cell is called forespore, which contains cytoplasm and DNA.
Stage 3.
- It is the engulfment of forespore. In this, the extension of membrane takes place to cover the future endospore. It develops a second membrane.
Stage 4.
- It is stage of accumulation of cortex in the space between two membranes.
Stage 5.
- It is concerned with formation of protein coat around the cortex.
Stage 6.
- It is concerned with maturation of endospore. During this, spore coat is formed completely and there is an increase in refractivity.
Stage 7.
- It is the last stage of sporulation. In this stage, the endospore is released from the cell. The release is caused by action of several autolytic enzymes, which digest cell wall of sporulating bacterium.
Germination of Spore :
- Transformation of dormant spore into vegetative cells called germination.
- A group of scientists claimed in then report about the revival of 25 - 40 million years old spore from the gut of an extinct bee.
- It suggests that spore well-designed structure that can remain dormant for hundreds of thousands of years.
- Dormancy may be long, but return to vegetative stage is very rapid. It occurs in three stage.
- Activation
- Germination
- Out growth
Activation
- It is conditioning of the spores to germinate when nutrients are available.
- It can be achieved by:
- Heat treatment, at sub lethal temperatures in which spores are kept at 60 to 70°C for several minutes.
- Storage of spore suspension at 4°C or at room temperature.
Germination
- It is concerned with the breakdown of spore dormancy. Changes taking place during germination stage include .....
- Swelling. rupture or absorption of spore coat.
- Loss of calcium dipicolinate and cortex components.
- Increase in stainability.
- Loss of refractivity.
- Loss of resistance to heat and other stresses.
- Release of spore components.
- Increase in metabolic activity.
- Loss of calctum dipicolinate.
- It can be triggered by metabolites or nutrients like sugars and amino acids.
Out growth
- It is concerned with emergence of cell from the spore.
- It is associated with visible swelling due to water uptake. Spore protoplast makes new components like RNA. protein and DNA. It turns Into an active bacterium.
