Endospores: structure and formation

  • 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:

   a) activation,

           b) germination 

           c) 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.

 

Image result for formation of endospore                           Image result for formation of endospore

 

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.

 

Image result for endospore                             Image result for formation of endospore

 

  • 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

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