BACTERIAL ENDOSPORES – FORMATION, STRUCTURE AND FUNCTIONS

BY: Reddy Sailaja M (MSIWM031)

Introduction

Microorganisms have the ability to adapt themselves to the changing conditions prevailing in the environment. Factors that influence microorganism’s survival could be physical, chemical or environmental.
Some microorganisms go in search of favorable conditions for survival, while some will become dormant till the favorable conditions arrive.
One such mechanism adapted by certain gram-positive bacteria is the development of ‘endospores. Gram positive bacteria, especially genera, Bacillus and Clostridium have the ability to form endospores in response to harsh conditions, nutrient deprivation in particular.
When there is starvation due to nutrient deprivation, these bacteria produce most resistant and dormant ‘endospore ‘structures that preserve cell’s genetic composition to with stand the harsh assaults like high temperature, desiccation, UV radiation, chemical and enzymatic damage.
Moreover, endospores are the most resistant form of “spores” or “cysts” produced by many bacteria and are resistant to most of the antibiotics. Altogether, endospores are resistant and dormant structures of life survival forms of bacteria and fight against harsh environments.

Formation of endospore

Figure 1: Development of Endospore

Bacillus subtilis is the model organism used to study and understand the development of endospore during the process is called sporulation.
It takes many hours to complete endospore formation. Morphological changes that occur during this process are used as markers to classify stages of endospore development. Stage I is that when the bacterial cell is under favorable conditions.
Under unfavorable conditions, bacterial cell initiates endospores formation by asymmetric cell division and is called Stage II. Asymmetric cell division results in the formation of a larger mother cell and a smaller forespore (or pre-spore) with septum in between them.
Even though, these two cell types in stage II has varied developmental fates, intercellular communication system harmonize cell specific gene regulation by influencing specialized sigma factors in the cells.
In stage III, peptidoglycan present in the septum gets dissolved and the mother cell engulfs forespore, which becomes a cell within a cell.
In the stages IV+V, cortex and the spore coat layers are formed around the forespore, leading to the production of endospore specific compounds.
In the stages VI+VII, further dehydration and the maturation of the endospore happens. Finally, the mother cell dies by apoptosis (also called programmed cell death) and the endospore is release into the environment and remains dormant until favorable conditions prevail.

Endospore structure

Endospore structure comprises of multiple layers of coats that resist against harsh surrounds. The following table details various layers (from outer to inner), their compositions and functions.

Endospore layer	Composition	Function
Exosporium	Carbohydrates, proteins and lipids	Gives hydrophobic character to the endospore and is responsible for endospore pathogenicity
Spore coat	Coat proteins cross-linked with disulfide bonds	Acts as primary permeability barrier and allows only smaller molecules like germinants
Outer membrane	–	Not known
Cortex	Peptidoglycan without teichoic acids with low cross linking	Structural differences in the peptidoglycan of cortex and germ cell wall allow selective degradation of outer protection, germination of endospore and transformation of germ cell wall into vegetative cell.
Germ cell wall	Peptidoglycan
Inner membrane	Similar to cell membrane composition. Germinant receptors	Varied fluidity and permeability and decreased mobility of the membrane lipids make the structure highly impermeable to the molecules including water, protecting core. Germinant receptors allow binding of germinants and begin germination and vegetative growth.
Core	Bacterial DNA, RNA, ribosomes, essential enzymes, small acid-soluble spore proteins (SASPs), Dipicolinic acid	Dehydrated state protects enzymes and heat resistance. SASPs protect DNA from destructive chemicals and enzymes by forming shield. SASPs also function as carbon and energy source during germination into vegetative cell. Dipicolinic acid also protects endospore’s DNA against harsh environment.

Figure 2: Structure of endospore

Mechanism of sporulation

Sporulation of endospores is under the control of five kinases, namely KinA, KinB, KinC, KinD and KinE that act under phosphorelay signal transduction mechanism.
Each of these kinases gets activated based on specific environmental stimuli. Under a specific kind of environmental stimulus, one of the five sensor kinases undergoes autophosphorylation at conserved histidine residue by an ATP dependent reaction through a protein called Spo0F. Then the Spo0F transfers the phosphate to Spo0B, that act as a mediator and delivers signal to Spo0A.
Spo0A further positively regulate genes necessary for sporulation and negatively regulate genes required for vegetative growth.

Figure 3: Mechanism of sporulation

Functions of endospores

Endospores mainly resist harsh conditions like high temperatures, disinfectants, radiation, etc.
Endospores are reported to survive for millions of years. For example, viable endospores were isolated from gastrointestinal tract of a bee that was embedded in amber around 25-40 years ago.
Dipicolinic acid and SASPs are crucial in protecting core of the endospore that contains genetic material.

Infectious diseases caused by endospores

In spite of defensive mechanism, endospores also transmit some infectious diseases as follows:

i)Anthrax – caused by Bacillus anthracis endospores when inhaled, ingested will germinate under suitable conditions and spread the infection

ii)Botulism – Caused by Clostridium botulinum. Spreads through unprocessed food and infect

iii) Tetanus – caused by Clostridium tetani. Spread through anaerobic wounds and cause infection.

Other infectious diseases like gas gangrene and pseudomembranous colitis are also popular.