What Are Endospores?
Bacteria, like Clostridia and Bacilli, form endospores to endure harsh environmental conditions such as nutrient deficiency.
The bacteria recognize and adapt to the changes in the environment. When nutrients are not available, some bacteria look for nutrients at various places or they might generate enzymes to make use of alternative sources.
Certain low G+C gram positive bacteria develop a strategy for survival through the formation of endospores, which is a complex developmental process as a response to the scarcity of nutrients. The bacteria create a highly resistant and dormant cell cover to protect the genetic material inside the cell during extreme stress. Many bacteria, like Myxococcus and Azotobacter, form cysts, while some bacteria, like Clostridium and Bacilli form spores. The endospores formed by low G+C gram positive bacteria are observed to be more resistant to the harsh environmental conditions. The low G+C gram positive bacteria are those pathogenic bacteria that have less than 50 percent of guanine and cytosine in their DNA.
Endospores protect the bacterium from environmental stress which might otherwise destroy or kill the bacteria. Environmental stress includes high ultraviolet radiation, enzymatic destruction, high temperature, desiccation, gamma radiation, and chemical damage. The resistant nature of endospores is marvelous, and they are highly significant as the bacterial endospores are not readily killed or destroyed by antimicrobial treatments.
Endospores are different from bacterial vegetative cells structurally, metabolically, and functionally. Many of the endospores sustain even for 10,000 years or more. As these endospores are viable for longer durations and are durable to stress conditions, the bacteria that produce these endospores are pathogenically bad.
The bacterial endospores cannot be killed by boiling them at 100 degree C, nor by washing them with alcohol or hydrogen peroxide. However, they can be destroyed by autoclave processes. Autoclaving is done to sterilize the materials at high pressure and at a temperature of 121 degree C for 15-20 minutes. Endospores are visible under electron and light microscopes. They cannot be stained by the bacterial stains used in gram staining.
The adaptive mechanism, called sporulation, developed by the bacteria for perseverance and propagation during stressful environmental conditions, is the formation of endospores.
Importance of Endospores
The prevalence of endospore formation is mostly observed among rod shaped bacteria including Clostridium and Bacillus. Endospore formation is also observed in Planosarcina ureae, Spirillum amyliferum, and Spirillum praeclarum.
When growth in the microbial culture has stopped, formation of endospores begins. The sensitivity of bacteria to the environment triggers the formation of a forespore, which suddenly forms with higher dimensions than that of a completed full spore.
The exact reason for the high resistant capacity of endospores to extreme temperatures, chemicals, and radiation is not properly decoded yet. However, several explanations were proposed in this regard by the scientific community.
Characteristics of endospores include that they are visible under light and electron microscopes and that they cannot be stained by usual bacterial stains like safranin. Endospores can be visualized by using Malachite Green and the procedure for staining is called Schaeffer-Fulton staining.
Sporangium is the mother cell that gives rise to endospores. Sporangium differs from vegetative cells. Based on the position of endospores in the mother cell, the sporangium can be classified as central pore, subterminal spore, terminal spore, or terminal spore with swollen sporangium. Aerobic and anerobic bacteria can produce endospores. Archae bacteria do not create endospores.
The process of endospore formation is termed as sporogenesis or sporulation. When the bacteria encounter lack of nutrients, they enter the sporulation phase. The core of the endospore becomes extremely dehydrated. Endospore formation in the bacteria happens in seven stages represented as Stage-I to Stage-VII.
Endospore Formation Steps
Formation of Axial Filament — The genetic material of the bacterial cell gets aligned exactly at the center of the cell.
Development of Septa — The plasma membrane invaginates inwards into the cell interior and creates a septum called forespore septum. This forespore septum formation leads to the division of a tiny portion of the DNA from the rest of the genetic material in the cell.
Forespore Engulfment — The newly formed immature spore is entirely engulfed by the mother cell while its membrane grows continuously. With the engulfment of the forespore, the spore is now surrounded by two layers of plasma membrane and an intramembrane space.
Development of the Cortex — The cortex is formed in between the membranes in the intermembrane space. Calcium and dipicolinic acid get accumulated in this stage.
Development of the Protein Coat — A layer of protein is formed as a coat on the cortex of the newly developed spore.
Maturation of Spores — At this stage, the core becomes extremely dehydrated and the bacterial cell becomes metabolically inactive.
Enzymatic Release of Endospores — The sporogonium, or spore mother cell, is destroyed by the enzymes, releasing the endospores.
Endospore germination involves the dormant endospore transforming into a metabolically active vegetative cell. When there is sufficient food available in the environment, germination of spores takes place. Just like sporulation, germination also takes place in a complex way.
Germination of endospore happens in three stages. They are:
- Activation: The spores must be activated before germinating even if they are kept in nutrient rich medium. Activation prepares the endospore to germinate. Activation can revert to inactivation if the environmental conditions are not conducive. Activation can be done through heat shock as well.
- Germination: The dormant state of the endospore will be broken by germination. Endospore germination occurs by the swelling of the spore, rupture of the spore coat, lack of resistance to heat and radiation, lack of refractility, release of spore components, and increase in metabolic acidity of spores. Germination of endospores is irreversible. If the spore encounters any unfavorable conditions for its sustenance, it cannot get back to the quiescent stage and it dies. Germination occurs when the activated spores are exposed to nutrients, like amino acids and sugars.
- Outgrowth: This is the third stage of endospore germination and the spore develops completely from the spore coat. The spore protoplast gets exposed to the surroundings and develops into an active vegetative cell.
Endospores have a unique structure that is specific for enabling the bacteria to be resilient to the environment. The outer coat is proteinaceous, which provides enzymatic and chemical resistance. Below the protein coat, there is a thick layer called the cortex. The cortex provides resistance to high temperatures. The germ cell wall exists below the cortex.
After the germination of endospore, the peptidoglycan layer will become the bacterial cell wall. The inner membrane, present below the germ cell wall, is a bigger permeability barrier for many damaging chemicals. The core of the endospore is at the center of it and is in a dehydrated state. The core contains the DNA, ribosomes, and dipicolinic acid. Dipicolinic acid is a chemical that is unique to the endospore. It occupies around ten percent of the dry weight of the spore and plays a major role in keeping the spore dormant.
Acid soluble, small proteins are also found in the endospores and they are known to condense and bind the DNA tightly. They are also responsible for resistance to UV light and DNA damaging chemicals. Chemicals, like toxin crystals, and structures, like outer layer made of glycoprotein known as exosporium and stalks, are present in the endospores.
Bacillus form endospores when there is deficiency of carbon or nitrogen in the growth medium. Bacillus subtilis spore coat is a multilayered protective structure that comprises of more than 70 different proteins. Apart from its protective nature, spore coats drive the spore germination and define the kind of association the spores can have with different surfaces in the environment.
Fluorescence microscopy followed by high-resolution image analysis has generated spatially scaled coat protein network showing that the coat has developed into four separate layers. This has led to the discovery of the spore crust, which is the outermost layer of the coat in B. subtilis.
Analysis of spore coat assembly revealed two features about the coat morphogenesis. First, it revealed the formation of scaffold cap on the spore surface with the proteins. Second, the encasement of spores is successive. The coat assembly is regulated at the transcription stage by sequential expression of coat genes and at the translation level by the coordination of directing the coat proteins to form specific coat layers and spore encasement.
Bacterial endospores are dormant, resistant spores produced by gram-positive bacteria during unfavorable environmental conditions and they help the bacteria to endure the harsh environmental conditions. The most studied endospore forming bacterial genera include Bacillus and Clostridium. The endospore producing mother cell is called sporangium. Position of endospore in the mother cell differs. The inner core of the endospore is covered by layers called the cortex, spore coat, and exosporium. The core of the spore is surrounded by core wall and it houses the DNA.
- The bacterial core comprises of dipicolinic acid combined with calcium.
- The cortex layer is made of peptidoglycan surrounded by a thick proteinaceous spore coat.
- The formation of forespore, engulfing of forespore followed by the accumulation of dipicolinic acid, constitute the first few steps in endospore formation.
- These are followed by the formation of the cortex, the proteinaceous spore coat, maturation, and the release of the endospore from the mother cell.
The released endospore germinates to give rise to the vegetative bacterial cell. Germination of endospore occurs in three stages. They are:
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What is endospore made of?
The outer exosporium is proteinaceous. Below this layer is a thick layer called cortex, which is made of peptidoglycan. Below the cortex, there is a layer called germ cell wall, which acts as a barrier for many unwanted chemicals. The core is in dehydrated state, housing DNA and ribosomes along with dipicolinic acid.
What is an endospore and its function?
Endospore is a tough, dormant, resistant form of bacteria that is developed to endure the hard environmental conditions by the bacteria. Endospore protects the bacterium from the environmental stress like lack of nutrients.
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