The bacterial spores they are prokaryotic cellular structures of resistance produced by bacteria to withstand and survive in unfavorable environmental conditions. Once the environmental conditions are favorable, they give rise to a new individual.
The synthesis of bacterial spores occurs through a process called sporulation. Sporulation is stimulated by the scarcity of nutrients (carbon and nitrogen sources) in the environment in which some types of bacteria live.
In all ecosystems in the biosphere we find many different species of bacteria, most of which produce spores. Bacteria are prokaryotic organisms, that is, they are characterized by being microscopic unicellular, lacking internal membranous organelles and having a cell wall, among other things..
Our general knowledge about bacteria is that they are the causative agents of many diseases (etiological agents), since they are capable of proliferating in other living organisms, causing infections and destabilizing the functioning of their physiological system..
Therefore, many of the sterilization protocols in human industries, mainly in the pharmaceutical, agricultural and food industries, focus on reducing, controlling and exterminating these microorganisms and their spores from the surfaces of products that are marketed through of the different markets.
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Bacterial spores are extremely resistant structures, designed to withstand different types of environmental “stress” such as high temperatures, dehydration, solar radiation or the presence of different chemical compounds..
Bacterial spores are typically enveloped by 6 different layers; although these may vary depending on the species of bacteria. These 6 layers are:
Inside each bacterial spore are all the essential components to form an individual similar (if not identical) to the one that gave rise to it. These elements include:
The spores are considered a form of asexual reproduction, since many times the conditions become unfavorable due to an excessive growth of the population and the bacteria that perceive the stimulus of the scarcity of resources begin sporulation.
It is important to understand that all bacterial spores give rise to individuals genetically identical to the one that gave rise to them, so considering them a form of asexual reproduction is perfectly valid.
In the innermost part of bacterial spores is the protoplast, also known as the "spore nucleus" or the "germ cell".
The external structure of the spore is designed with the primary function of protecting the protoplast, which contains the cytoplasm, DNA and RNA molecules, proteins, enzymes, cofactors, ions, sugars, etc., which are necessary for the metabolic maintenance of the bacteria.
The first layer that surrounds the protoplast is the cell membrane, made up of lipids and proteins. It has many specialized structures in the interaction with the outermost covers, in order to perceive the environmental stimuli received by them..
Both the inner and outer cell wall, which are the layers that precede the cell membrane, have the typical structure of the bacterial cell wall: they are composed mainly of the heteropolysaccharide called peptidoglycan (N-acetyl glucosamine and acid N-acetyl muramic).
Covering the walls we just mentioned is the cortex, which is made up of large peptidoglycan chains (45-60% muramic acid residues).
On the cortex are the inner and outer layer of bacterial spores, made up of proteins with specialized functions to deactivate enzymes and toxic chemical agents that could damage the spore. Two of the most abundant enzymes in this layer are superoxide dismutase and catalase..
Exosporium (which is not produced by all species) is made up of proteins and glycoproteins that block the access of large proteins such as antibodies, for example. This layer is believed to be found in bacteria that depend on a pathogenic character to survive..
Spore formation begins when bacterial cells activate the genetic pathway that controls sporulation functions. These genes are activated by means of proteins and transcription factors that detect environmental changes (or the transition from "favorable" to "adverse")..
The classical model used to study the formation of a bacterial spore is the one observed in Bacillus subtilis, which is subdivided into 7 stages. However, the formation of spores in each bacterial species has its peculiarities and may involve more or fewer steps.
The stages of sporulation can be easily appreciated, with the help of a microscope and by observing cells growing in nutrient-deficient environments. We can describe these stages roughly as follows:
The cell increases its cytosolic volume at least three times in a relatively short period.
Concomitant with the increase in cytosolic volume, the genome of the bacterium is duplicated by mitosis. At the end of mitosis, the “maternal” genome aligns itself towards one of the poles of the cell, while the “daughter” or resulting genome aligns towards the opposite pole..
The cell membrane begins to constrict very close to the pole where the “daughter” genome produced during mitosis is located. This contraction ends up isolating the resulting genome from the rest of the cell's cytosol..
The segment formed by the constricted cell membrane is reinforced by another portion of the cell membrane, forming a double membrane and giving rise to an immature spore known as “forespora”.
The bacterial cell increases the production of muramic acid residues. These are directed towards the surface that covers the forespora, generating an additional layer of protection. Once the formation of this layer is completed, the forespora is called exospora.
Increases in muramic acid production are also oriented to form two layers of a composition of peptidoglycan similar to that of the bacterial cell wall. These two layers will form the inner and outer covering of the exospore and transform it into an endospore..
The last step in sporulation or spore formation is release. The cell wall, membrane and all coatings of the "mother" cell are lysed and release the mature endospore into the environment..
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