Certain species of bacteria produce spores, either within the cell (endospores) or external to the cell (exospores). The spore is a metabolically dormant form which, under appropriate conditions, can undergo germination and outgrowth to form a vegetative cell. The organism survives in a dormant state for longer period of starvation and other adverse conditions.
In simple terms the process of endospore formation is called sporulation or sporogenesis. It occurs normally when growth of bacterium ceases due to lack of nutrients.
Therefore sporulation can be thought of as repackaging a copy of bacterial DNA into a new form that contains very little water, has greatly reduced metabolic activity, does not divide, and has a restructured, highly impermeable, multilayered envelope.
Sporulation in bacteria is not a means of reproduction because one vegetative cell forms a single endospore, which cannot divide and after germination remains one cell.
Process of Sporulation
The process of sporulation or sporogenesis normally begins when growth ceases due to lack of essential nutrients or in other words when conditions become unfavourable for growth and multiplication. The entire process may be divided into following stages:
- The nuclear material reorganizes itself axially from end to end.
- Spore septum is formed which is the ingrowth of plasma membrane. The spore spectrum becomes a double layered membrane that surrounds a copy of bacterial chromosome and little cytoplasm.
- A structure entirely enclosed within cell is formed which is called as forespore. The mother cell, which contains the endospore, is called the sporangium.
- Between the two membrane a thick layer of peptidoglycan is laid down. A thick coat of protein is formed around this structure that provides resistance to endospore.
- All endospores contain DNA, small amount of RNA, ribosomes, enzymes and large amount of organic acid called dipicolinic acid. As much as 15% of the spore’s dry weight consists of diplicolinic acid complexed with calcium ions.
- Therefore, dehydration of the protoplast appears to be more important in providing heat resistance, whereas the calcium- dipicolinic acid complex may be conferring stability to the spore nucleic acids.
- These cellular components are important for resuming metabolism in later stages.
- The sporogium now begins to deteriorate. Lytic enzymes destroy the sporangium releasing the spore. The free spore can remain dormant yet viable for thousands of years. The whole process of sporulation requires only about 10 hours.