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Flagellar motility

Anton van Leeuwenhoek first described bacterial flagella in 1675, and many bacterial species elaborate at least one flagellum to move through liquid environments (Berg and Anderson, 1973). A typical bacterial flagellum consists of three parts: the basal body, the hook, and the flexible filament (Berg and Anderson, 1973). The flexible filament, which can be made up of 20,000 flagellin subunits, can extend out nearly three times the length of the bacterial cell (Macnab, 2003). In some bacteria, such as Vibrio cholerae and Helicobacter pylori, the flagellar filament may have an additional external sheath. In the stomach pathogen H. pylori, this external sheath is an extension of the outer membrane and is thought to protect the acid-labile flagellar structure from attack by stomach acid (Geis et al., 1993).

The flagellar filament is attached to a rigid “hook” that provides stability to the filament and connects it to the basal body, which is the apparatus that harbors the components that provide the energy for flagellar rotation and is the point of elaboration for the filament (Macnab, 1977; Silverman and Simon, 1974; Turner et al., 2000). The basal body is composed of multiple rings that span from the cytoplasm to the membrane (to the outer membrane in Gram-negative bacteria) and are named based on their cellular locations. Formation of the basal body and subsequent secretion of subunits to build the flagellum follow an ordered sequence that is dictated by tight regulation of genes, as well as protein-embedded signals that lead to hierarchical secretion of subunits over time (Osterman et al., 2015; Chevance and Hughes, 2008; Erhardt et al., 2010; Paul et al., 2008; Macnab, 2003; Homma et al., 1990; Hirano et al., 1994).

The innermost ring of the basal body is called the C ring based on its location in the cytoplasm. In E. coli, this ring is composed of three proteins, FliM, FliN, and FliG that, along with the stator proteins MotA and MotB, drive motor rotation. Deletion of one of the genes encoding these proteins leads to mutant strains that produce nonrotating flagella (Kojima and Blair, 2004; Minamino et al., 2008). Much like jet engine propellers, bacterial stator proteins provide the force that rotates the flagellum. Bacterial stators make use of cations to generate the energy required to power the flagellar machinery (Manson et al., 1977; Paul et al., 2008; Berg, 2003). Most bacteria studied to date use protons that are generated from the electron transport chain (Berg, 2003). Unlike the majority of bacteria that require proton motive force, Vibrio species pump sodium ions across the membrane to power flagellar rotation (Atsumi et al., 1992; Kojima et al., 1999). The ability to elaborate and power the flagellum allows for bacterial movement, while the direction of flagellar rotation determines the type of movement that occurs.

Some bacteria, such as V. cholerae, harbor a single flagellum at one pole of the cell, while others harbor multiple flagella that can be arranged in multiple ways along the bacterial cell surface (McCarter, 2004; Hranitzky et al., 1980). Some bacteria such as E. coli and Salmonella spp. elaborate 5-10 flagella that are either bundled together (lopho- trichous) or are expressed at various locations along the cell surface (peritrichous), depending on the mode of motility engaged by the bacteria (Macnab, 1977).

There are distinct types of flagellum-based motility: swimming, swarming, and tumbling. Counterclockwise rotation of polarly placed flagella leads to a forward propelling motion known as swimming (Harshey, 2003). This type of forward motion is thought to occur in response to a chemoattractant that is sensed by the bacteria. Forward motion is also observed in swarming motility. Swarming allows for bacterial group motility across a solid surface (Harshey, 2003) and requires expression of flagella along the cell periphery (Partridge and Harshey, 2013). Swarming not only requires the same flagellar rotation to generate forward motion as in swimming, but must also overcome frictional forces and attract water to the surface to allow group movement across the surface (Partridge and Harshey, 2013). Bacteria overcome these physical hurdles by releasing surfactants to decrease the friction around them (Turner et al., 2010; Be’er and Harshey, 2011). If the flagella rotate in a clockwise fashion rather than counterclockwise, this rotation will cause the cell to fall or tumble backward due to the lack of coordination in the flagellar filaments (Darnton et al., 2007). This disordered, backward movement of the flagella leads to a tumbling motility that allows the bacterial cell to change directions (Macnab and Ornston, 1977).

 
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