lecture 9 - Microbial Cell Structures Learning Goals...

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Microbial Cell Structures
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Learning Goals Understand the different types of bacterial cell walls, Gram positive, Gram negative, S layers; describe their properties Realize that some microbes do not have cells walls Learn about inclusions, list some examples and their function Understand what the periplasm is, and that enzymatic reactions occur in it Learn about surface structures that microbes have, e.g., flagella, pili, fimbriae, and glycocalyx
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Cell envelope: General properties Protects again osmotic shock Rigid and gives cell its shape Structure is unique to prokaryotes many compounds found in the cell wall are found no where else Can confer pathogenic properties
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The cell wall Gram negative Gram positive
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Comparisons Gram positive Gram negative Single layer Two layers Peptidoglycan Peptidoglycan, Outer membrane
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Peptidoglycan (aka Murein ) Polymer of two sugars N-acetyl glucosamine (NAG) N-acetyl muramic acid (NAM) cross-linked by amino acid bridges
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What is diaminopimelic acid?
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Peptidoglycan polymer
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What are the differences between crosslinks?
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Physical arrangement Gram positive heavily cross-linked many layers (20-80 nm thick) 50% of cell weight Gram negative not as cross-linked 15-20% of cell wall is peptidoglycan Peptidoglycan is not a barrier to solutes
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The Gram-positive cell wall Teichoic acids Glycerol
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A closer view Surface of a Bacillus . Transmission E.M. C=Capsule; S=S-layer; P=Peptidoglycan. Pasteur Institute.
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The Gram-negative cell wall
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Braun’s lipoprotein of Gram-negative bacteria
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The Outer Membrane: Structure of LPS Lipid A Two NAG Fatty acid chains Core polysaccharide distinct for each species Often contains KDO, heptose, glucose and glucosamine O-antigen highly variable KDO = 2-keto-3-deoxyoctonate
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What does KDO look like?
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Toxicity of LPS (endotoxin) Toxic to mammals, induces shock Symptoms Decrease in blood pressure leads to vascular collapse - death pyrogenic - fever inducing
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Outer membrane proteins (Omp) Fewer types than in cytoplamic membrane Porins are one class Allow passage of small molecules Outer membrane not barrier Omp = porins
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Cabib, E. et al. J. Biol. Chem. 2001;276:19679-19682 A , the linkage between chitin [ (1 T 4) NAG)] and (1 3)glucan. B , structure of a complex that contains all four components of the cell
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lecture 9 - Microbial Cell Structures Learning Goals...

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