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Download The outer membrane IS a barrier to large solutes such as glucose
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Other Extracellular Layers Outer membrane Capsule Sheath Cell Appendages Filamentous, small: Fimbriae, Pili, & Spinae Filamentous, large: Flagella Outer Membrane Most common in Gram negative bacteria, often associated with a fairly thin peptidogylcan layer A second lipid bilayer, but NOT water and ion impermeable (permeable largely due to the presence of channel proteins called porins). The outer membrane IS a barrier to large solutes such as glucose. Often is an asymmetric bilayer, where the outer leaflet contains the unusual lipid LPS (LipoPolySaccharide). LPS can be shed from the outer membrane and is a major endotoxin Cell Layers in a Gram Negative Bacterium Lipopolysaccharide (LPS) Structure Capsules A thick layer of secreted polysaccharide which surrounds the cell generally a defensive structure, often involved in pathogenicity Protein Jackets Like capsules are secreted by the cell and built around the cell Much more complex than capsules mostly made of protein Often associated with multicellular bacteria Fimbriae, Pili, & Spinae Frimbriae & Pili both refer to filamentous projections from the cell surface made of protein (Frimbriae is the more general term) There is an ongoing attempt to classify different types of pili: Type I --- adhesive Type II --- involved in general secretion of proteins to the outside of the cell Type IV --- defined by assembly pathway and so on . . . Spinae, as the name implies are spine-like projections composed of protein often used to increase the cell’s surface area Two Types of Bacterial Motility Swimming: ability to move through liquids --- usually employing a spiral propeller (flagellum) --- some bacteria can move over surfaces with flagella --- all bacterial flagella are genetically similar (Horizontal Gene Transfer) Surface (Gliding) Motility: movement over surfaces --- cells lack flagella --- gliding cells generally cannot swim --- multiple systems have arisen among different groups of bacteria Styles of Flagellation Swimming Bacteria equatorial (R. sphaeroides) peritrichous (E. coli, S. typhimurium) polar (A. serpens) polar in at least one archaeon (Halobacterium salinarium) All bacteria have essentially the same flagellar genes (homologous) The Archea have an analogous structure that is not genetically similar to the bacterial flagellum Flagella Used for bacterial locomotion (usually swimming) requires tremendous energy input (up to 20% of cellular energy budget for multiply flagellated cells) can serve as a model system for intracellular signaling Flagellar Factoids -“flagellum” is a misnomer from early microscopy, the filament is helical and appears to move in a wave-like manner as it rotates, it is actually quite rigid - each flagellar filament is composed primarily of one protein, flagellin (FliC), in about 1 million copies, flagellin is also an important antigen for the immune system - the flagellar filament is a passive structure, basically a propeller - the flagellum is driven entirely at it’s base by a complex of proteins that function as a proton driven rotary motor The flagellar motor is fueled by the trans-membrane ion gradient ~ 300 rev/sec outer membrane peptidoglycan (hundreds/rev) inner membrane MotA/B Torque Generators (Stator Complex) CheY- P The flagellar motor is reversible CCW: run CW: tumble E. coli responds to chemical gradients by biasing its random walk No stimulus Gradient of chemoattractant [Asp], e.g. Chemotaxis - the ability to sense and respond to extracellular concentration gradients of solutes Input Chemoreceptors CheY – P - CW signal - 3 sec. lag bacterial cell Output: motor bias, CCWCW rotation
