By nature, spores are durable and can survive in less than ideal conditions. All fungi produce spores; however, not all bacteria produce spores! Furthermore, fungal spores and bacterial spores are different in how they function and how they are produced. Fungal spores are used to help fungi reproduce asexually some fungi can also reproduce sexually.
As an organism becomes mature or stressed, the fungal cells begin to produce spores as a means to propagate. Fungal spores are single cells that will float through the air, looking for a favorable environment to begin developing. Fungal spores are very hearty and can survive for years in unfavorable conditions. However, compared to bacterial spores they are less durable. The center of the endospore, the core, exists in a very dehydrated state and houses the cell's DNA, ribosomes and large amounts of dipicolinic acid.
Small acid-soluble proteins SASPs are also only found in endospores. Other species-specific structures and chemicals associated with endospores include stalks, toxin crystals, or an additional outer glycoprotein layer called the exosporium.
The process of forming an endospore is complex. The model organism used to study endospore formation is Bacillus subtilis. Endospore development requires several hours to complete. Key morphological changes in the process have been used as markers to define stages of development. As a cell begins the process of forming an endospore, it divides asymmetrically Stage II. This results in the creation of two compartments, the larger mother cell and the smaller forespore.
These two cells have different developmental fates. Intercellular communication systems coordinate cell-specific gene expression through the sequential activation of specialized sigma factors in each of the cells.
Bacterial spores are highly resistant to. An endospore is structurally and chemically more complex than the vegetative cell. It contains more layers than vegetative cells. Resistance of Bacterial spores may be mediated by dipicolinic acid, a calcium ion chelator found only in spores. When the favorable condition prevails, i. A mature endospore contains a complete set of the genetic material DNA from the vegetative cell, ribosomes and specialized enzymes.
Mature endospores are released from the vegetative cell to become free endospores. When the free endospores are placed in an environment that supports growth, the endospores will revert back to a vegetative cell in a process called germination.
It should be noted that unlike the process of binary fission observed with vegetative cells, endospore formation is not a reproductive process but a process of differentiation that provides the bacteria with a mechanism for survival. Usually they are a flexible, but strong shell — a thin continuous protein basal layer, which has an outer, hair, and inner, crystalline layers. The subtle features of exosporia can vary under different growth conditions and in different types of bacteria [68,].
The use of modern methods of molecular biology has significantly expanded the understanding of the biochemical structure and spatial organization of the exosporial structures of the Bacillus spp family and Clostridium spp. Figure 2. The exosporium of these bacteria is a thin and flexible shell structure, which is usually much larger than the dense endospore located inside [5,14].
The main structural element of exospores in different types of bacteria is a thin crystalline basal protein layer [12,13,80]. It turned out that this glycoprotein plays a significant role in protecting spores from phagocytosis [21,78,79].
In addition, it was recently shown that it mediates the mechanism of immune inhibition, which contributes to the preservation of spores in the lungs of mice [1,83].
The region between the basal layer of the exosporium and the outer shell of the endospore is called the intermediate space [26,60] and is approximately nm [67]. In some places, the basal layer is located in close proximity to the outer layer of the endospore membrane. The exosporium basal layer has a thickness of approximately 12 to 16 nm and consists of two sublayers approximately 5 nm thick [26,67].
The basal layer has a crystalline structural organization with 6-fold symmetry and a periodic interval of 7 nm. The outer surface of this structure consists of a series of hexagonal concave cups in the form of honeycombs, with open ends oriented outwards [17]. This channel structure provides the barrier properties of exosporia [17,67,84].
Hairy filaments of exospore, usually consisting of BclA protein, have a length of 14 to 70 nm and cover the entire surface of the outer shell of the basal layer [67,81,82,86,87]. Recent studies have shown that, unlike other bacteria of the B. Synthesis of Exosporium and its Biochemical Structure. The biochemical structure of exosporium, which differs significantly from the structure of endospores, has been the subject of many reviews [13,26,88,92].
The initiation of synthesis occurs at the central pole of the spore. First, the embryo of the future exospore appears in the form of a small layered structure in the spore-forming mother cell. The synthesis ends at the opposite pole of the spore after attaching the basal layer of the exospore to the outer shell of the spore. Studies conducted in recent years on B. In the study of mutant isolates of B. Boydston et al. Steichen et al. Of these proteins, CotY and ExsY are involved in exosporia biosynthesis at the initial stages of the formation of the basal layer base, while ExsFA and ExsFB - at the final stages [74,88].
Both of these proteins some authors put an equal sign between them are necessary at the stage of formation of filaments of the fleecy layer [23,67,82,90]. Among the proteins involved in the biosynthesis of exosporium, BclA glycoprotein deserves special attention.
Brahmbhatt et al. In the experiments, the specific rBclA antigen enhanced the protection of mice infected with B. The same proteins determine the biochemical structure of the exosporium [66,]. When considering the sequence of stages of exosporium biosynthesis, the temporal dynamics of the appearance of specific glycoproteins and their inclusion in the process is of interest.
These proteins first appear as a high molecular weight protein complex, the assembly of which occurs in the cytoplasm of the mother cell sporangia long before these proteins are located in places of developing exosporium. The appearance of these proteins in the sporulating cell in the form of monomers occurs an hour before the start of exosporia biosynthesis [89].
Manetsberger et al. These proteins form part of the outermost layer of B. It was revealed that these proteins are a key component of the basal layer of the B. However, the complexity of the biochemical structure of exosporia does not end there. Previous studies conducted by V. Thompson et al. Exosporium structural proteins determine its properties [45]. The outer surface of the basal layer consists mainly of BclA glycoprotein, which plays a major role in protecting spores from macrophages [84,87].
Recently, it was shown that this protein mediates the mechanism of immune inhibition [86,97], which experimentally contributed to the preservation of spores in the lungs of mice. Enzymes that make up exospores, inosine hydrolase, and alanine racemase [85,88,94] can inhibit premature spore germination [98,99]. However, the main protein that determines the structure of endosporium a long time was unknown.
Jiang et al. According to the authors, the same protein determines the hexagonal crystal structure of exosporium by disulfide binding of cysteine residues within individual ExsY polypeptide chains and its subunits [18,28,63].
These disulfide bonds underlie the ability of ExsY and its homolog CotY to self-assemble into an ordered crystal lattice of high symmetry for stable assembly of exosporia in all species of Bacillus spp. As is known, adhesion is an important factor of virulence in these types of bacteria [8,9,17]. It would seem that the question of basic proteins, which determines the structure of exosporium, is closed, however, in P.
They found that during sporulation, all analyzed C. Despite the lack of experimental evidence, the authors suggested that each morphotype apparently plays a different role in the pathogenesis of the infection associated with C. It is noteworthy that, despite the ultrastructural differences, the outer fleecy layer was found in both exospore morphotypes [9,10,69]. The same group of researchers, using a gel-free approach for analysis of the exosporium layer and combined extraction methods, revealed the presence of proteins in the exosporium layer [10,70].
In this group of proteins, special attention was paid to collagenlike exospore BclA proteins, which form hair filament structures [69,70,73]. Given the fact that BclA2 and BclA3 are widely present in most strains of these bacteria, it is possible that vaccines developed on their basis can provide immune protection against clinically significant isolates.
The absence of these proteins reduces the pathogenicity of C. These proteins are structural and turned out to be highly immunogenic [32,64,,]. Experimental data indicate that they are of interest for immune biotechnologies as potential antigens for the development of new types of vaccines.
For example, immunization of mice with CdeM protein demonstrated an IgG-specific immune response []. Vaccination of hamsters with the CdeM protein of the recombinant strain C. This is striking given that the CdeM protein is unique to C. Further experiments, for example, with the inclusion of adjuvants in the composition of the CdeM vaccine in order to more quickly form a pronounced and long-lasting specific immune response, may increase its protective effect [,].
Immunization of mice with cysteine rich CdeC protein caused a significant IgG immune response after three immunizations with IP recombinant CdeC []. It was found that CdeC is an immunogenic protein [3,64].
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