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Quantifying the Magnetic Advantage in Magnetotaxis

M. J. Smith ^*, P. E. Sheehan ^*, L. L. Perry , K. O'Connor , L. N. Csonka , B. M. Applegate  and L. J. Whitman ^*

^* Chemistry Division, Naval Research Laboratory, Washington, District of Columbia 20375; and Department of Food Science, and Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907

Correspondence: Address reprint requests to L. J. Whitman, Tel.: 202-404-8845; E-mail: whitman{at}nrl.navy.mil.

Magnetotactic bacteria are characterized by the production of magnetosomes, nanoscale particles of lipid bilayer encapsulated magnetite, that act to orient the bacteria in magnetic fields. These magnetosomes allow magneto-aerotaxis, which is the motion of the bacteria along a magnetic field and toward preferred concentrations of oxygen. Magneto-aerotaxis has been shown to direct the motion of these bacteria downward toward sediments and microaerobic environments favorable for growth. Herein, we compare the magneto-aerotaxis of wild-type, magnetic Magnetospirillum magneticum AMB-1 with a nonmagnetic mutant we have engineered. Using an applied magnetic field and an advancing oxygen gradient, we have quantified the magnetic advantage in magneto-aerotaxis as a more rapid migration to preferred oxygen levels. Magnetic, wild-type cells swimming in an applied magnetic field more quickly migrate away from the advancing oxygen than either wild-type cells in a zero field or the nonmagnetic cells in any field. We find that the responses of the magnetic and mutant strains are well described by a relatively simple analytical model, an analysis of which indicates that the key benefit of magnetotaxis is an enhancement of a bacterium's ability to detect oxygen, not an increase in its average speed moving away from high oxygen concentrations.

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