Surface Sensing Sample Clauses

Surface Sensing. P. mirabilis in liquid exists in its vegetative form: short, peritrichously flagellated rod-shaped cells. Mechanosensing following surface contact triggers swarmer cell differentiation. The flagella and LPS are two structures used by P. mirabilis to sense surfaces. Much like V. parahaemolyticus and other members of the γ-proteobacteria, the flagella of P. mirabilis act as mechanosensors. Flagella freely rotate in liquid. However, by increasing the viscosity of the environment, adding anti-flagellin antibodies, or adding cells to a solid surface, the inhibition of flagellar rotation creates torque on the motor, triggering differentiation (25, 257). Correct flagellar assembly is required, as mutants of the major flagellin subunit xxxX and the secretion chaperone flgN do not differentiate (149, 258). Additionally, mutants of flagellar basal body proteins responsible for torque generation have been shown to constitutively elongate under non-permissive conditions. These include mutants in FliG, a switch complex protein of the rotor; FliM, which forms the MS-ring within the rotor; and FliL, a protein that stabilizes the MotAB stator. All of these proteins are essential for powering the flagellar filament (257, 259). The behavior of these mutants results from a defect in mechanosensing, the consequences of which are cells that behave as though they are perpetually in contact with surfaces. Overall, the P. mirabilis flagellum acts as mechanosensor by sensing and responding to external forces on the flagellar motor. The O-antigen subunit within the LPS is also important for surface sensing. Mutants in genes involved in LPS and O-antigen biosynthesis can swim but do not swarm due to inefficient cell elongation (258). Mutants in waaL encoding O-antigen ligase and wzz, which determines O-antigen chain length, do not activate the flagellar gene cascade (250). Overall, cell membrane perturbations mediated by LPS and O-antigen following surface contact are sensed by regulatory networks which affect swarmer cell differentiation. These regulatory networks include the Umo proteins and the Rcs phosphorelay which are discussed below. Surface contact is sensed by the membrane proteins UmoA-D (upregulator of master operon) which activate expression of the flagellar master operon flhDC. UmoA is outer membrane associated, UmoC and UmoD are periplasmic, and UmoB is associated with the inner membrane. Mutants in umoA-D have reduced flhDC transcript levels, with no flhDC transcript ...
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