VMC 211: General Veterinary Microbiology (1+1)

SPORES

• Endospore: Spore produced within the cell
• Exospore: External to the cell

Endospores

• Endospores are unique to bacteria. They are thick walled highly retractile bodies produced by Bacillus, Clostridium, Sporosarcina and few other genera.
• Shape and location of the spore within the cell vary depending on the species – central, sub terminal and terminal.
• Examples
  • terminal -Clostridium tetani
  • central - Bacillus cereus
  • subterminal - Bacillus subtilis

• Sometimes the endospore can be so large the cell can be distended around the endospore, this is typical of Clostridium tetani .
  • Endospores can not be stained by regular staining due to the impermeability of the endospore wall to dyes and stains. The rest of a bacterial cell may stain but the endospore is left colourless. Endospores can be stained by the Schaffer and Fulton stain, which stains endospores green and bacterial bodies red.
  • Endospores are resistant to dessication, heat, radiation, disinfecting agents and staining. The degree of heat resistance of endospores varies with bacterial species.
  • Most spores can resist 90 C for atleast 10 minutes. Dehydrated state and dipicolinic acid (DPA) may be responsible for heat resistance.
  • DPA is unique to endospores. DPA constitutes 10-15% of the spore’s dry weight. DPA occurs in combination with large amount of calcium.
  • Calcium dipicolonate may play a role in heat resistance. DPA synthesis and uptake of calcium occur during advanced stages of sporulation.
  • Spores are usually produced by cells growing in rich media which are approaching the end of active growth.

Sporulation

• When environmental conditions are become unfavourable sporulation is started. It takes about eight hours. The DNA is replicated and a membrane wall called spore  septum begins to form between it and the rest of the cell.
• The plasma membrane of the cell surrounds this wall and pinches off to leave a double membrane around the DNA. The developing structure is now known as a forespore.
• Calcium dipicolinate is incorporated into the forespore during this time. Next the peptidoglycan cortex forms between the two layers and the bacterium add a spore coat to the outside of the forespore.
• Sporulation is now complete, and the mature endospore will be released when the surrounding vegetative cell is degraded.

Germination

• Formation of vegetative cells from spores is called germination. During germination endospores loose their resistance to heat and staining.
• Outgrowth occurs, characterized by synthesis of new cell material and development of the organism into a new cell.

Exospores

• Methylosinus genus forms exospores by budding at one end of the cell. They are heat and dessication resistant but they do not contain DPA.