Bacterial Spore: structure, types, sporulation and germination

July 25, 2017 Gaurab Karki Bacteriology, Microbiology 0




Bacterial Spore: structure, types, sporulation and germination

  • Spore is metabolically dormant structure produced during unfavorable condition by the process called sporulation
  • Sporulation occur during late log phase or early stationary phase
  • Under favorable condition spores germinate to give vegetative cell.
  • Size: 0.2 µm
  • Spore are resistant to nutrition starvation, temperature, extreme pH, antibiotics etc

Structure of endospore:

An endospore has following layers

  1. Exosporium
  2. Spore coat
  3. Cortex
  4. core

Exosporium:

  • It is the outermost layer made up of protein that encloses spore coat.
  • In some bacterial spore, exosporium is made up of polysaccharide and lipid.

Spore coat:

  • It is thick double layered covering that encloses cortex
  • Spore coat consists of spore specific protein, mainly contains cysteine and hydrophobic amino acids. Due to presence of these aminoacids,spore are resistant to adverse environmental condition.

Cortex:

  • Inside the Spore coat, there is cortex made up of loosely arranged peptidoglycan layer.
  • Inner layer: comprises about 20% of peptidoglycan, it is tightly arranged
  • Outer layer: it is loosely arranged, it can be hydrolysed during spore germination.it comprises alalnine (55%), tetra-peptide (15%) and muramic lactum (30%).

Core:

  • It is the innermost part of spore
  • It is also known as spore protoplast
  • Core consists of core wall, cytoplasmic membrane, cytoplasm, nucleoid, ribosomes and other cellular materials.
  • Core contains (10-25%) water so, the cytoplasm is gel like
  • It contains high amount of calcium and dipicolinic acid in the form of calcium dipicolinate (10-15% by dry weight).
  • Core also contains high percentage of small acid soluble protein (SASP).
  • SASP is synthesized during sporulation and it binds to DNA in core and protect it from potential damage caused by UV radiation, desiccation and drying.
  • In addition, SASPs also provides nutrition and energy for spore germination.

Types of bacterial spore

1. Endospore:

  • It is produced within the bacterial cell.
  • Bacteria producing endospore are: Bacillus, Clostridium, Sporosarcina etc

2. Exospore:

  • It is produced outside the cell
  • Bacteria producing exospore: Methylosinus

Sporulation

  • During unfavorable condition, vegetative cell converts into spore by the process known as sporulation
  • Sporulation can be divided into several stages. In Bacillus subtilis, entire process of sporulation takes 8 hours to complete from stage 0 to stage VII

Stages of sporulation:

Stage 0

  • Normal conditions of a vegetative cell

Stage I: Axial filament formation stage

  • In this stage bacterial chromosome become thread like known as axial filament
  • Axial filaments attached to cytoplasmic membrane by mesosome
  • Elongation of cell take places
  • PHBA is the reserved food material in Bacillus spp is utilized in sporulation.

Stage II: forespore formation

  • Asymmetric cell division occurs
  • Cell membrane forms septum near one end which encloses a small portion of DNA forming forespore

Stage III: engulfment of forespore

  • Mother cell membrane grow around the forespore engulfing it.
  • Fore spore now has two membrane layer

Stage IV: synthesis of exosporium

  • Chromosome of mother cell disintegrates
  • Exosporium synthesis occurs
  • Forespore starts Forming primodial cortex between two membrane.
  • Dehydration of cell

Stage V: synthesis of dipicolonic cacid

  • Production of SASPs and dipicolinic acid occurs
  • Incorporation of calcium ions with dipicolonic acid occur forming calcium dipicolonate
  • Further dehydration of cytoplasm
  • Formation of coat layer

Stage VI: maturation

  • Maturation of endospore

Stage VII: release of endospore

  • Cell lysis and release of endospore

Spore germination

  • Endospore remains dormant for years. But under favorable conditions each endospore germinates to give rise to a vegetative cell.

Spore germination involves 3 process.

  1. Activation
  2. Germination
  3. Outgrowth

Activation of endospore:

  • The germination of bacterial spore do not occur even when the environment is favorable unless it is first activated.
  • At first the Spore coat must be damaged by heating for several minutes.

Germination:

  • The activated spore initiates germination after binding of effector molecules.
  • Binding of effectors molecules activates autolysis that destroy peptidoglycan of cortex.
  • After destruction of peptidoglycan, water is taken up and calcium dipicolinic acid is released.

Outgrowth:

  • After uptake of water swelling of spore occurs.
  • Along with swelling, synthesis of DNA, RNA and proteins also occurs.
  • A small germ cell emerges out after breaking the Spore coat and begins to grow into vegetative cell.

Bacterial Spore: structure, types, sporulation and germination