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Topic_4 - Topic 4 Fundamentals of...

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Unformatted text preview: Topic 4 Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: Understand cell structure with respect to cell membrane composition and motility. The cytoplasmic membrane surrounds the cell. It is made up of a phospholipid bilayer. Make sure that you understand the structure and composition of phospholipid bilayer, as illustrated in Figure 4.4 and 4.5. The cytoplasmic membrane also includes integral membrane proteins (see Figure 4.5). There are important differences between the lipids in Bacteria and the lipids in Archaea. As illustrated in Figure 4.7, archaeal lipids have ether linkages, while bacterial lipids have ester linkages. Please review the structure of lipids in Section 3.4. Functions of the Cytoplasmic Membrane (Figure 4.9): • Permeability Barrier  ­ ­ only small, uncharged, hydrophobic molecules can pass through by diffusion (see Table 4.2). Protein Anchor  ­ ­ transport, generation of energy, chemotaxis. Generation of proton motive force. • • Transport proteins are required for the accumulation of solutes within the cell against the concentration gradient (carrier ­mediated transport). Specific transport proteins transport specific molecules or specific classes of molecules. In this course, you will not be responsible for knowing the different types of transport events and transport systems, but I encourage you to read about them in the textbook if you are interested. The cell wall of bacteria is made of a thin sheet called peptidoglycan. It prevents the cell from lysing due to turgor pressure (see Figure 4.21). Peptidoglycan is only found in bacteria. Gram ­negative cells have an outer membrane exterior to the cell wall, while Gram ­positive cells do not have an outer membrane. Peptidoglycan is composed of a glycan backbone (alternating molecules of N ­ acetylglucosamine and N ­acetylmuramic acid) connected by peptide cross ­links. Figures 4.18 and 4.19 will help you to understand the structure of peptidoglycan. Gram ­positive bacteria have teichoic acids, a type of acidic polysaccharide, embedded in the peptidoglycan layer. The structure of a particular teichoic acid is shown in Figure 4.20. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Although archaeal organisms do not have peptidoglycan walls, many have a wall constructed of a similar material called pseudomurein (or sometimes pseudopeptidoglycan). Pseudomurein differs from peptidoglycan in that its backbone is composed of alternating molecules of N ­acetylglucosamine and N ­acetyltalosaminuronic acid which are connected by ß ­1,3 linkages rather than ß ­1,4 linkages as in bacterial peptidoglycan. Comparing Figures 4.18 and 4.25 will help you to understand the differences between peptidoglycan and pseudomurein. Gram ­negative organisms have a more complex cell envelope (Figure 4.23) that includes an outer membrane, exterior to the peptidoglycan cell wall. The space between the inner and outer membranes, which includes the peptidoglycan, is called the periplasm. The outer membrane consists of a phospholipid bilayer and lipopolysaccharide (Figure 4.22). Porins are proteins that span the outer membrane and allow small molecules to cross membrane. Many prokaryotes possess flagella, which they use to propel themselves. This ability to propel themselves is called motility. Some bacteria have polar flagella, which are attached at the end of the cell. Other bacteria have peritrichous flagella, which are distributed around the cell. See Figure 4.47 for a depiction of flagellar structure. As you can see, flagella are complex, elaborate structures. The long filament is made up of subunits of flagellin protein. Another type of protein forms the hook, which connects the filament to the basal body. The basal body is a motor that is embedded in the cytoplasmic membrane and cell wall, and rotates. Note that the flagellar rotation, like a propeller, rather than pulling like an oar, is what propels the cell. Counterclockwise rotation propels the cell forward in a run, while clockwise rotation causes the cell to tumble or propels the cell backward. This is shown in Figure 4.50. Why are bacteria motile? Motility must confer a selective advantage; otherwise it would not be worth the effort to expend the energy that motility requires. It turns out that motile bacteria can respond to the presence of favourable or detrimental chemicals by moving toward or away from them. When bacteria respond in this way to chemicals, it is called chemotaxis. Bacteria sense a chemical gradient in a temporal manner, rather than a spatial manner. The Figure 4.53 shows how the chemical gradient can influence the duration of runs, resulting in biased net movement up or down the gradient. Besides chemotaxis, there are other types of taxis: • • phototaxis  ­ light aerotaxis  ­ oxygen • Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld osmotaxis  ­ osmotic strength Many types of structures and cell inclusions are found only in certain types of bacteria. Some of these are described in Section 4.9, 4.10 and 4.11. • • • fimbriae  ­ aid cells to adhere to surfaces pili  ­ conjugation, attachment to host cell glycocalyx  ­ polysaccharide layer outside cell, attachment to host cells, protection from host immune system, resistance to desiccation polyhydroxyalkanoate deposits  ­ intracellular carbon and energy store polyphosphate  ­ intracellular reserves elemental sulfur  ­ intracellular granules magnetosomes  ­ intracellular magnetite crystals gas vesicles  ­ cell buoyancy • • • • • Certain types of bacteria are able to differentiate into endospores, which are resistant to heat, radiation, acids, drying, and many chemicals. Endospores can remain dormant for very long periods of time. There are even recent claims of successfully reviving endospores that are over 250 million years old (see page 94 ­95, and the original paper in the lecture folder). The stages of endospore formation are presented in Figure 4.43. These stages were defined by analysis of mutants that were blocked in sporulation. Sporulation is induced by conditions of nutrient limitation. ...
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