16 - PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 16...

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PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 16 The Spirochetes
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• Spirochetes are gram-negative, motile, tightly coiled Bacteria , which are typically slender and flexuous in shape. •Top: Spirochaeta stenostrepta ; left: S. plicatilis ; left bottom: Borrelia burgdorferi . • They are morphologically unique and form a major phylogenetic lineage in the Bacteria . • Widespread in aquatic environments and in animals. • Some cause diseases, including syphilis, an important human sexually transmitted disease. • Spirochetes are classified into eight genera primarily on the basis of habitat, pathogenicity, phylogeny, and morphological and physiological characteristics.
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The spirochete cell is made up of a protoplasmic cylinder and moves by means of structures called endoflagella Above right: nine cross-sectioned endoflagella can be recognized (dark dots beneath the OM). http://www.old.uni-bayreuth.de/departments/emikroskopie/acker40.htm
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Structure of the prokaryotic flagellum in gram-negative Bacteria • Flagella are not straight but are helically shaped. • The filament is composed of subunits composed of the protein flagellin. • The base of the flagellum is called the hook. It consists of a single type of protein and functions to connect the filament to the motor portion. • A flagellum functions by rotation, much like a propeller in a motor boat. • The motor is anchored in the cytoplasmic membrane and cell wall. • The motor consists of a small central rod that that passes through a series of rings. • In gram-positive bacteria, only the inner pair of rings are present. • The Mot and Fli proteins are anchored in the cytoplasmic membrane, the latter of which functions to reverse the direction of rotation in response to intracellular signals.
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The “proton turbine” model of flagellum rotation The energy required for rotation of the flagellum comes from the proton motive force, but the exact mechanism is not completely understood. A model has been proposed in which protons flowing through channels in the stationary Mot protein exert electrostatic forces on helically arranged charges on the rotor proteins. Attractions between positive and negative charges would cause the basal body to rotate as protons flow through the Mot protein.
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Forward motion in imparted by all flagella rotating counterclockwise in a bundle. Clockwise rotation causes the cell to tumble. Cells change direction by reversing
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16 - PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 16...

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