The behaviour of magnetotactic bacteria in an AC magnetic field is studied both the-oretically and experimentally. The role of thermal fluctuations is studied by simulating the Brownian dynamics. The origin of the magnetic moment coupling to the body of bacterium is studied and it is shown that besides the coupling due to the magnetodipolar interaction of magnetosomes the contribution due to the flexibility of cytoskeletal proteins may be important. The behaviour of magnetotactic bacteria is studied experimentally in rotating and AC magnetic fields. The magnetic moment of bacterium is determined by measuring its retardation angle in a rotating field. It is illustrated experimentally that magnetotactic bacteria in an AC magnetic field orient themselves perpendicularly to the field lines as predicted by the theoretical model. Experimental results on the magnetotactic bacteria behaviour in the AC field are described quantitatively using a model, which considers the action of a small steady field, and the strength of the magnetic moment coupling to the body of bacterium is determined.
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