| Capsules, slime layers and S layers |
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Capsules and slime layers (also known as glycocalyx) are layers of polysaccharides lying outside the cell wall; they protect the bacteria from phagocytosis, desiccation, viral infection, and hydrophobic toxic materials such as detergents; they also aid bacterial attachment to surfaces and gliding motility a. Capsules are well organized b. Slime layers are diffuse and unorganized |
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S layers are regularly structured layers of protein or glycoprotein observed in both bacteria and archaea, where it may be the only structure outside the plasma membrane; they protect against ion and pH fluctuations, osmotic stress, hydrolytic enzymes, or the predacious bacterium Bdellovibrio |
| Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial attachment to surfaces (fimbriae) or to other bacteria during sexual mating (pili) |
| Flagella and motility |
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Flagella are threadlike locomotor appendages extending outward from the plasma membrane and cell wall; they may be arranged in various patterns: |
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Monotrichous-a single flagellum |
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Amphitrichous-a single flagellum at each pole |
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Lophotrichous-a cluster (tuft) of flagella at one or both ends |
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Peritrichous-a relatively even distribution of flagella over the entire surface of the bacterium |
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Flagellar ultrastructure: |
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The flagellum consists of a hollow filament composed of a single protein known as flagellin. |
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The hook is a short curved segment that links the filament to the basal body, a series of rings that drives flagellar rotation. |
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Flagellar synthesis involves many genes for the hook and basal body, as well as the gene for flagellin. New molecules of flagellin are transported through the hollow filament so that the growth of the flagellum is from the tip, not from the base. |
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The mechanism of flagellar movement appears to be rotation; the hook and helical structure of the flagellum causes the flagellum to act as a propeller, thus driving the bacterium through its watery environment |
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Counterclockwise rotation causes forward motion (called a run) |
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Clockwise rotation disrupts forward motion (resulting in a tumble) |
| Procaryotes can move by other mechanisms; in spirochetes, axial filaments cause movement by flexing and spinning; other procaryotes exhibit gliding motility-a mechanism by which they coast along solid surfaces; no visible structure is associated with gliding motility |
