- Some species of bacteria particularly those of Bacillus and Clostridium, develop structures called endospores.
- ‘Endo’ as prefix means within and endospores are highly differentiated cells which are able to tolerate harsh environment of heat, chemicals and radiations.
- They are highly resistant resting phase which can survive in a dormant state through a long period of starvation or other adverse environmental conditions.
- Endospore forming bacteria are found most commonly in soil.
- Endospores enable the organism to endure difficult times and work as survival structures.
- They exist as intermediate phage in life cycle of bacteria: vegetative cell → endospore → vegetative cell.
- They are the structures that help in dispersal of an organism by various means like wind, water or through animal gut.
- A vegetative cell gets converted into a non-growing, heat resistant structure during the sporulation process.
- When there is sufficient amount of essential nutrients then the growth of cells keeps on going actively.
- At this stage, cells do not sporulate. The sporulation occurs only when the growth ceases due to exhaustion of essential nutrients.
- Thus, some species of bacteria like Bacillus, which is a typical endospore former, ceases vegetative growth and begins sporulation.
- This occurs when the carbon or nitrogen nutrient become limited.
- An endospore can remain in dormant state for many years.
- But, it can convert itself back to the vegetative cell relatively rapidly.
- Three processes are involved in its conversion from spore to vegetative form.
- They are:
- The freshly formed endospore is heated for several minutes at an elevated but sub-lethal temperature for accomplishing the activation process.
- Then the activated endospores are provided with a suitable environment like presence of sufficient nutrients such as amino acids (a good trigger of endospore germination) to germinate.
- The germination process involves loss of microscopic refractility of the endospore, increased ability to be stained by dyes, and loss of resistance to heat and chemicals.
- The third stage that is the outgrowth stage involves visible swelling due to water uptake and synthesis of new RNA, proteins and DNA.
- The cell then comes out from the broken endospore after which it begins to grow.
- The cell then remains in vegetative form unless and until environmental condition gets worsened to trigger sporulation.
- It stands as a refractile structure when observed through light microscope.
- It is impermeable to many dyes due to which it is seen as unstained region within cells that have been stained with methylene blue.
- Special stains and procedures must be used to stain the endospore.
- Malachite green has been used to stain the endospore along with the steaming process in the classical protocol of endospore staining.
- The structure of endospore differs distinctly from that of the vegetative cell if observed through electron microscope.
- It consists of many layers which makes its structure more complex than the vegetative form.
- Exosporium is the outermost layer which is a thin protein covering.
- Spore coats composed of layers of spore-specific proteins are present within the outer layer.
- Cortex is situated just inner to the spore-coat made up of loosely cross-linked peptidoglycan.
- Inner to the cortex is another structure that is called core containing the core wall, cytoplasmic membrane, cytoplasm, nucleoid, ribosomes and other cellular essentials.
- Thus, endospore is totally different in the kind of structures found outside the core wall from the vegetative form.
- Dipicolinic acid is present in spore form and absent in vegetative form.
- This substance is located in the core of endospores of all endospore- forming bacteria.
- Endospores are also rich Ca++ which is present in combined form with dipicolinic acid.
- The complex of calcium-dipicolinic acid forms about 10% of the dry weight of endospore.
- This complex helps the spore form to reduce water availability within the endospore, thus helping to dehydrate it.
- The complex also intercalates (inserts between bases) in DNA due to which the DNA gets stabilized to heat denaturation.
- Also, the cytoplasm of the core of endospore has gel like consistency due to less water content than the vegetative form.
- Dehydration plays vital role in providing the endospore the heat resistance ability of macromolecules.
- Some bacterial endospores survive the high temperature of 1500C.
- But the temperature of 1210C that is standard for microbiological sterilization kills the endospores of most species.
- Boiling has also no any negative effect on the endospores.
- Resistance to chemicals is also due to the dehydration condition of the endospore which keeps the enzymes of the core in inactive form.
- The pH of the core is about 1 unit lower than the vegetative cell cytoplasm.
- Small acid soluble proteins (SASPs) are also present in the core of endospore in addition to other factors.
- These proteins are made during the sporulation process which helps to bind to the DNA tightly.
- This binding helps to protect it from potential damage from UV radiation, dessication and dry heat.
- It also functions as a carbon and energy source for the outgrowth of the vegetative cell from the endospore during germination.
Endospores: structure and formation