Understanding the motility of bipolar magnetotactic bacteria
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Magnetotactic Magnetospirillum magneticum AMB-1 cells are amphitrichous bacteria that swim by coordinating the rotation of two flagella attached at opposite poles of their spirillum-shaped cell body. The swimming modes of such cells - run, reverse, tumble or pause - depend on the combination of rotational directions of the two bipolar flagella. We present a mathematical model of an amphitrichous bacterium to investigate its swimming mechanisms. The cell body is modelled as a rigid spiral tube with approximately one turn. Each flagellum, attached at opposite poles, is intrinsically left-handed and described using the Kirchhoff rod model. The rotation of flagellar motors serves as the driving force for the cell motility in fluid, and the hydrodynamics of the cell is governed by the incompressible Stokes equations, assuming the force- and torque-free conditions at all times. Our findings suggest that appropriate material and geometrical properties of flagella and the cell body are necessary to reproduce the experimentally observed swimming modes of motility.