Endospores: structure and formation

Introduction

• 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.

Endospore formation

• 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:

1. activation,
2. germination 
3. growth.

• 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.

Endospore structure

• 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.

References: 

i) https://www.easybiologyclass.com/bacterial-endospore-definition-characteristics-structure-and-its-formation/

ii) http://www.ndvsu.org/images/StudyMaterials/Micro/Bacterial-Endospores.pdf

Endospores: structure and formation