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Category: Bacterial Pathogenesis
The Spore Coat, Page 1 of 2
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The coat varies considerably in width among species. In Bacillus subtilis, where the coat is relatively wide, it is just less than 200 nm in width, and its multilayered organization is unmistakable by transmission electron microscopy (TEM). Importantly, the number of coat layers and the presence or absence of appendages extending from the coat surface vary among species. This interspecies variation and differences in complexity drew attention as soon as spores were imaged at high resolution, and in the decades since ( 1 – 7 ). The coat is readily distinguished from the cortex (see reference 178 ) because of its higher electron density. In a large subset of species, the spore also possesses an additional layer surrounding the coat, called the exosporium ( Fig. 1 ; see also references 8 and 9 ).
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Thin-section TEM analysis of spores from diverse species. Spores were prepared as described in McKenney et al. ( 13 ). Images in the top row were fixed using ruthenium red. Other images were conventionally fixed. Images are not to scale; each image was sized to facilitate comparison. Two images of Bacillus amyloliquefaciens are shown (one showing a section along the long axis, the other showing a section along the short axis) to point out the thick caps of coat at the poles. The difference in thickness between the two caps is a consistent feature of this species. Two images of Brevibacillus laterosporus are also shown to emphasize the variation in morphology of the distinctive structure (indicated with a brown bracket) associated with the coat. The mother cell envelope, which is still present in these two spores, is indicated with a green bracket. The image of Clostridium difficile is taken from Semenyuk et al. ( 142 ). The crust (Cr), outer coat (OC), and inner coat (IC) are indicated in the image of Bacillus subtilis in the upper left. The coat and, where it is present, the exosporium are indicated with blue and red brackets, respectively. The image of Bacillus megaterium is courtesy of Dr. Joel Bozue at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID).
Thin-section TEM analysis of spores from diverse species. Spores were prepared as described in McKenney et al. ( 13 ). Images in the top row were fixed using ruthenium red. Other images were conventionally fixed. Images are not to scale; each image was sized to facilitate comparison. Two images of Bacillus amyloliquefaciens are shown (one showing a section along the long axis, the other showing a section along the short axis) to point out the thick caps of coat at the poles. The difference in thickness between the two caps is a consistent feature of this species. Two images of Brevibacillus laterosporus are also shown to emphasize the variation in morphology of the distinctive structure (indicated with a brown bracket) associated with the coat. The mother cell envelope, which is still present in these two spores, is indicated with a green bracket. The image of Clostridium difficile is taken from Semenyuk et al. ( 142 ). The crust (Cr), outer coat (OC), and inner coat (IC) are indicated in the image of Bacillus subtilis in the upper left. The coat and, where it is present, the exosporium are indicated with blue and red brackets, respectively. The image of Bacillus megaterium is courtesy of Dr. Joel Bozue at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID).
Model of spore coat assembly during B. subtilis sporulation. In the left column, we list the stages of sporulation as they appear by TEM, phase-contrast microscopy, or fluorescence microscopy in the presence of a membrane stain. The center column contains diagrams of spore coat morphogenesis. Layers of the spore coat are color coded (cyan = basement layer; yellow = inner coat; blue = outer coat; maroon = crust). In the right column, we list the stages of spore coat assembly. DPA, dipicolinic acid. Modified from McKenney et al. ( 21 ). See text for details.
Model of spore coat assembly during B. subtilis sporulation. In the left column, we list the stages of sporulation as they appear by TEM, phase-contrast microscopy, or fluorescence microscopy in the presence of a membrane stain. The center column contains diagrams of spore coat morphogenesis. Layers of the spore coat are color coded (cyan = basement layer; yellow = inner coat; blue = outer coat; maroon = crust). In the right column, we list the stages of spore coat assembly. DPA, dipicolinic acid. Modified from McKenney et al. ( 21 ). See text for details.
Classes of coat proteins based on localization kinetics. Spore coat genes are displayed according to their localization on the B. subtilis chromosome with the origin of replication (oriC) on top. Genes whose expression commences before engulfment, under the control of σE, are inside the circle; genes whose expression begins after engulfment, under the control of σK, are outside the circle. Classes are color coded (red = class 1; brown = class 2; orange = class 3; purple = class 4; blue = class 5; turquoise = class 6). Genes encoding morphogenetic proteins are underlined.
Classes of coat proteins based on localization kinetics. Spore coat genes are displayed according to their localization on the B. subtilis chromosome with the origin of replication (oriC) on top. Genes whose expression commences before engulfment, under the control of σE, are inside the circle; genes whose expression begins after engulfment, under the control of σK, are outside the circle. Classes are color coded (red = class 1; brown = class 2; orange = class 3; purple = class 4; blue = class 5; turquoise = class 6). Genes encoding morphogenetic proteins are underlined.
B. subtilis coat proteins (strain 168)
B. subtilis coat proteins (strain 168)
C. difficile coat proteins (strain 630)
C. difficile coat proteins (strain 630)