**5. The movement of magnetotactic bacteria**

Several experimental observations show that magnetotaxis functions together with aerotaxis, determining the so-called magneto-aerotaxis [8, 67]. Basically, two different behaviors have been identified in magneto-aerotaxis: polar magnetotaxis, that consists in the North-seeking or South-seeking behaviors in the search for the better oxygen concentrations; and axial magnetotaxis, in that case, MTB move in the magnetic field direction but without preferential sense. MTB from natural samples always present polar magnetotaxis. Axial magnetotaxis has been observed only in cultured MTB.

MTB are easily identified because of their response to the inversion of the local magnetic field direction: after the inversion bacteria swim following the new magnetic field direction. It can be stated that magnetic field inversions stimulate MTB to swim, making them a model for the study of microorganism swimming. Bacteria swim in the low Reynolds number regime, where viscous forces and torques act to null the resultant force and torque [68]. In that regime, microorganisms swim following an helical trajectory [69] whose parameterization in Cartesian coordinates (x, y, z), considering the helix axis as the z axis, can be written as (Rcos(ωt), Rsin (ωt), Vt), where R is the helix radius, V is the axial velocity, and ω = 2π*f* being *f* the helix frequency. In the case of magnetotactic microorganisms, the helical trajectory of the multicellular magnetotactic prokaryote "*Candidatus* Magnetoglobus multicellularis" has been studied for two different applied magnetic fields (3.9 and 20 Oe) [70] and for magnetic fields from 0.9 to 32 Oe [55]. Those studies show that for spherical multicellular magnetotactic prokaryotes, the axial velocity V is about 90 μm/s, the radius R is about 8 μm for lower magnetic fields, and the helix frequency *f* is about 1.1 Hz. For uncultured magnetotactic coccus, the helical movement has been studied recently (data not published), in the presence of magnetic fields of about 0.7 Oe, and the helical parameters measured were: axial velocity of about 90 μm/s, radius of about 2.5 μm, and helix frequency of about 1 Hz. For other magnetotactic microorganisms, it has been observed that the 2D trajectory is similar to the projection of a 3D helix in the microscope focal plane (for example, see [19, 68, 71].

For the theoretical study of microorganisms, motion in the low Reynolds number regime is necessary to know all the forces and torques acting on the microorganism. Nogueira and Lins de Barros [68] developed a model in that regime, considering a spherical MTB with a single flagellum and a magnetosome chain aligned to the flagellum line. The equations to be considered are **F**flagella + **F**viscous = 0 and **τ**flagella + **τ**viscous + **τ**magnetic + **τ**body = 0. Using the appropriate expressions for the forces and torques in that model, they were able to calculate numerically the temporal evolution of the center of mass coordinates (x, y, z) and of the Euler's angles for the rigid body (θ, ϕ, ψ), being the trajectory similar to a cylindrical helix. In the other hand, Refs. [72, 73] studied the motion of nonspherical MTB, to include the effect of the bacterial body geometry on the viscous forces. Also, Yang et al. [73] studied MTB with two flagellar bundles. To do that, they calculated numerically the motion using the second Newton's law, considering all the forces and torques and calculating the appropriate inertial terms for the geometrical body form. They also studied the effect of the relative inclination λ between the magnetosome chain and the flagella. Those studies showed that when λ 6¼ 0, the velocity decreases when the magnetic field increases, effect also observed experimentally in the work by Pan et al. [74] when studying the circular movement of the MYC-1 strain. In that case, it was measured the velocity in the circular trajectory obtained in a bacteriodrome as function of the applied magnetic field, in the hope to obtain a growing Langevin curve as predicted by Kalmijn [54]. But they observed that the velocity decreases as the magnetic field increases, in the contrary of a Langevin curve. To explain this, they assumed that the magnetosome chain has an inclination relative to the flagellar bundle. Interestingly, it has been observed that some MTB strains have the magnetosome chain with different inclinations relative to the flagellar bundle, in some cases being almost perpendicular to it, not orienting the magnetic moment to the magnetic field direction during their swimming [73].

**Acknowledgements**

*Biology and Physics of Magnetotactic Bacteria DOI: http://dx.doi.org/10.5772/intechopen.79965*

**Conflict of interest**

**Author details**

**15**

Fernanda Abreu<sup>1</sup> and Daniel Acosta-Avalos<sup>2</sup>

\*Address all correspondence to: dacosta@cbpf.br

provided the original work is properly cited.

1 Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

2 Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brazil

We acknowledge the microscopy facility Unidade de Microscopia Multiusuário

\*

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

(UniMicro, UFRJ) and financial support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação de Amparo

à Pesquisa do Estado do Rio de Janeiro (FAPERJ).

The authors declare no conflict of interest.

The movement of magnetotactic microorganisms also depends on the presence of light, and the response depends on the wavelength and the intensity. This behavior has been studied mainly in multicellular magnetotactic prokaryotes. Negative photo response has been observed when they are illuminated with high-intensity UV light (365 nm), violet-blue light (395–440 nm filter) of about 80 W m<sup>2</sup> of intensity, and blue light (450–490 nm filter) of about 200 W m<sup>2</sup> of intensity. For longer wavelengths, no photo response was observed, and apparently long exposure to green light is lethal [70, 75]. That negative photo response is not observed when very low intensities are used. Photokinesis has been observed in multicellular magnetotactic prokaryotes, decreasing the velocity when illuminated with green light (517 nm, 0.46 W m<sup>2</sup> ) and increasing the velocity when illuminated with red light (628 nm, 0.16 W m<sup>2</sup> ), both respectively to the velocity observed when illuminated with blue light (469 nm, 0.8 W m<sup>2</sup> ) [76, 77]. Recently, De Melo and Acosta-Avalos [78] showed that the photokinesis in multicellular magnetotactic prokaryotes is related to the combined presence of monochromatic light and a constant magnetic field, and that it can be canceled in the presence of radio-frequency electromagnetic fields oscillating at the Zeeman resonance frequency associated to the constant magnetic field, showing the involvement of a radical pair mechanism, a very well-known magnetoreception mechanism used by migratory birds. Interestingly, magnetotactic microorganisms have the proper physical tools to sense the geomagnetic field with light. Perhaps, magnetotaxis and the radical pair mechanism are involved in a more elaborate magnetic sensing in MTB.
