• 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

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