Bacterial spore

In poor growth conditions some bacteria such as Bacillus and Clostridium produce resistant survival forms termed endospores. This process is known as sporulation. Bacterial spores are endospores in contrast to fungal spores, which are usually exospores. Unlike the spores of fungi, bacterial spores do not serve reproductive function. They are resistant to extreme environmental conditions such as high temperatures, dryness, toxic chemicals (disinfectants, antibiotics), and UV radiation. Once the endospore is formed, the vegetative portion of the bacterium is degraded and the dormant endospore is released. The endospore is able to survive for long periods of time until environmental conditions again become favorable for growth. The endospore then germinates, producing a single vegetative bacterium. Spores can be killed by sterilization methods such as autoclave and hot air oven. Some chemical disinfectants such as formaldehyde and ethylene oxide can also kill spores.

Mechanism of sporulation:
First the DNA replicates and the cell divides asymmetrically. A cytoplasmic membrane septum forms at one end of the cell. A second layer of cytoplasmic membrane then forms around one of the DNA molecules (the one that will become part of the endospore) to form a forespore. Both of these membrane layers then synthesize peptidoglycan in the space between them to form the cortex. Calcium dipocolinate is also incorporated into the forming endospore. A spore coat composed of a keratin-like protein then forms around the cortex. Sometimes an outer membrane composed of lipid and protein and called an exosporium is also formed. Finally, the remainder of the bacterium is degraded and the endospore is released. There is no metabolic activity until the spore is ready to germinate. Single vegetative cell gives rise to a single spore. Sporulation generally takes around 15 hours.

Germination:
Favorable growth conditions signal the process of endospore germination. Germination of a spore results in a break in the spore wall and the outgrowing of a new vegetative cell. The newly formed vegetative cell is capable of growth and reproduction. A single spore upon germination forms a single vegetative cell. Germination occurs in following steps:

Activation: Even in the presence of favorable conditions, the spore will not germinate until its protective spore coat is not damaged. Conditions such as heat, acidity, abrasion or compounds containing free sulphydryl groups activate the spore to germinate.
Initiation: once activated, the spore will germinate provided the environment is suitable. Different signaling effectors exist for different species. Binding of effector stimulates autolytic enzymes that degrade the peptidoglycan of cortex. Water is absorbed and calcium dipicolinate is released.
Outgrowth: once the cortex and outer layers is degraded, a new vegetative cell consisting of spore protoplast and its wall emerges. This is followed by active biosynthetic activity and process terminates with cell division.

The impermeability of the spore coat is thought to be responsible for the endospore's resistance to chemicals. The resistance of endospores is due to a variety of factors:
Calcium-dipicolinate, abundant within the endospore, may stabilize and protect the endospore's DNA. Specialized DNA-binding proteins saturate the endospore's DNA and protect it from heat, drying, chemicals, and radiation. The cortex may osmotically remove water from the interior of the endospore and the dehydration that results is thought to be very important in the endospore's resistance to heat and radiation. DNA repair enzymes contained within the endospore are able to repair damaged DNA during germination.

For more information on anatomy of bacterial cell, visit

www.microrao.com/micronotes/anatomy.pdf