BACTERIAL ENDOSPORES

 ENDOSPORES

·     Endospores are the resisting non-reproductive structures formed by bacteria for survival during adverse environmental condition.

·       Endospores were first discovered by John Tyndall. He discovered the heat resistant Endospores of bacteria which survived after heating at 100 °C.

·       Endospores can be variable in size and location within the cell.

·       Endospores are structurally, metabolically and functionally different from the Vegetative cells.

·    The completed endospore consists of multiple layers of resistant coats (including a cortex, a spore coat, and sometimes an exosporium) surrounding a nucleoid, some ribosomes, RNA molecules, and enzymes.

·       Endospores are commonly found in soil and water, where they may survive for long periods of time.

STRUCTURE OF ENDOSPORES

Bacterial Endospore (Source: www.easybiologyclass.com)

·       The structure of Endospore is very complex since they possess multilayer coverings.

·       The outermost layer of the spore is called as Exosporium which is relatively thin and delicate.

·       Beneath the Exosporium is a Spore coat composed of several layers of Proteins.

·    Spore coat is comparatively thick. The thickness of the Spore coat is one reason for the high resistance of Endospores towards heat, radiation and chemicals.

·       Inert to the Spore coat is Cortex. It is the thicker wall layer in the Endospore.

·       Cortex is very large and sometimes occupy as much as half of the Spore volume.

·       The Cortex is composed of Peptidoglycan. The peptidoglycan in the Cortex is less cross linked than that of Vegetative cells.

·       The innermost layer of the spore is called the Spore cell or Core wall.

·       Core wall covers the central protoplast or Core of the Endospores.

·       The Endospore core has a normal cell structure as that of a Vegetative cell.

·       The core contains Ribosomes and centrally placed Nucleoid (Genetic material DNA).

·       Unlike the Vegetative cells, the Core protoplast is metabolically inactive.

·       The core only contains about 10 – 25 % of water of the normal Vegetative cells.       

ENDOSPORE PROTEINS

(i) Dipicolinic acid

·       Located in the core of Endospores.

·       Calcium – Dipicolinic acid complexes reduces water available and helps to dehydrate spores.

·       Intercalates into the DNA and stabilizes it to heat denaturation.

(ii) Small Acid Soluble Proteins (SASPs)

·       Bind to the DNA in the core and protect it from damage.

·       Function as a carbon and energy source when forming vegetative cells to spore cells.

ENDOSPORE PRODUCING BACTERIA

·       Maximum endospore production is associated with Gram Positive bacteria (Gram positive rods) only and the Endospores are produced by few Gram negative bacteria only. 

·      A few genera of bacteria produce Endospore such as Clostridium sp. (Gangrene) and Bacillus sp. (Anthrax), both of them are Gram positive rods.

·  Other genera of bacteria, including Desulfotomaculum, Sporolactobacillus, Sporosarcina, Oscillospira and Thermoactinomyces are also able to produce Endospores.

·   Since not all bacteria form Endospores, so we can use the presence of Endospores as an identification factor.

LOCATION OF THE ENDOSPORES

·       Based on the location, Endospores are classified into four categories. They are

a)     Central spores or Equatorial appearing Spindle shape (Bacillus cereus and Clostridium bifermentans)

b)     Sub-Terminal spores appearing Club shape (Clostridium perfringens)

c)      Terminal oval spores resembling Tennis racket (Clostridium tertium)        

d)     Terminal spherical spores giving Drumstick appearance (Clostridium tetani)

Central

Sub - terminal

Terminal

Location of Endospores

FACTORS GIVING RESISTANCE TO ENDOSPORES

a)  Calcium-dipicolinate - Abundant within the endospore, may stabilize and protect the Endospore's DNA.

b)     Small acid-soluble proteins (SASPs) - Saturate the endospore's DNA and protect it from heat, drying, chemicals, and radiation. They also function as a carbon and energy source for the development of a vegetative bacterium during germination.

c)      Cortex - 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.

d)     DNA repair enzymes - Contained within the endospore are able to repair damaged DNA during germination.

SPORULATION OR SPOROGENESIS

·      The process of Endospore formation within a Vegetative cell takes several hours and is known as Sporulation or Sporogenesis.

·       Sporulation takes place during the Stationary phase of the Growth curve.

Sporulation or Sporogenesis

FUNCTIONS OF ENDOSPORES

·       Endospores are ultimately protection for the bacterial genome.

·       Spores form within the cell and contain a full copy of the bacterium’s genome.

·       Endospores provide resistance against 

a)     Heat

b)     Boiling

c)     Drying

d)     Antibiotics

e)     Low nutrient conditions

f)      Radiation

g)     Osmotic pressure

h)    Various chemical disinfectants

DIFFERENCE BETWEEN VEGETATIVE CELLS AND ENDOSPORES