156
Electromagnetic Fields in Biological Systems
Cryoelectron tomography of Magnetospirillum strains has revealed that the magnetosome chain is surrounded by a network of filaments that may be composed of MamK
given that the filaments are absent in the MamK mutant cells (Scheffel et al. 2006). The
process of assembly of MamK filaments is unknown. Pradel et al. (2006) proved the
authenticity of MamK filaments and showed that MamK exhibits linear distribution
inside Magnetospirillum sp. cells even in the area without magnetosomes. The MamK gene
alone is sufficient to direct the synthesis of straight filaments in E. coli, and one extremity
of the MamK filaments is located at the cellular pole. By using dual fluorescent labeling
of MamK, the authors found that MamK nucleates at multiple sites and assembles into
mosaic filaments. Time-lapse experiments revealed that the assembly of MamK filaments
is a highly dynamic and kinetically asymmetrical process. MamK bundles might initiate the formation of a new filament or associate to a preexisting filament. These results
demonstrated the mechanism of biogenesis of prokaryotic cytoskeletal filaments that are
structurally and functionally distinct from the known MreB and ParM filaments. In addition to positioning magnetosomes, other hypothetical functions of the MamK filaments in
magnetotaxis include anchoring of magnetosomes and involving in magnetic reception.
The classical model of polar magnetotaxis predicts that the field-parallel migration
velocity of magnetotactic bacteria increases monotonically with the strength of an
applied magnetic field. Pan et al. (2009) tested this model experimentally on magnetotactic coccoid bacteria that swim along helical trajectories. It turned out that the contribution of the field-parallel migration velocity decreases with increasing field strength
(0.1–1.5 mT). This unexpected observation can be explained and reproduced in a mathematical model under the assumption that the magnetosome chain is inclined with
respect to the flagellar propulsion axis. The magnetic disadvantage, however, became
apparent only in fields stronger than the geomagnetic field, which suggested that magnetotaxis is optimized under geomagnetic field conditions. It is therefore not beneficial
for these bacteria to increase their intracellular magnetic dipole moment beyond the
value needed to overcome Brownian motion in geomagnetic field conditions.
Magnetotactic bacteria benefit from their ability to form cellular magnetic dipoles by
assembling magnetosomes in chains as a means to navigate along geomagnetic field lines
on their way to favorable habitats. Faivre et al. (2010) studied the assembly of magnetosomes
by ferromagnetic resonance spectroscopy using Magnetospirillum gryphiswaldense cultured in a time-resolved experimental setting. The spectroscopic data showed that magnetic particle growth is not synchronized, the increase in particle number is insufficient to
build up cellular magnetic dipoles, and dipoles of assembled magnetosome blocks occur
when the first magnetite particles reach a stable single-domain state. The authors assumed
that these stable single-domain particles can act as magnetic docks to stabilize the remaining and/or newly nucleated superparamagnetic particles in their adjacencies. The authors
postulated that docking is a key mechanism for building the functional cellular magnetic
dipole, which in turn is required for magnetotaxis in bacteria.
3.4.3 Antibacterial Effects
László and Kutasi (2010) investigated the viability of the microbes Saccharomyces cerevisiae, Bacillus circulans, E. coli, Micrococcus luteus, Pseudomonas fluorescens, Salmonella
Electromagnetic Fields in Biological Systems
Cryoelectron tomography of Magnetospirillum strains has revealed that the magnetosome chain is surrounded by a network of filaments that may be composed of MamK
given that the filaments are absent in the MamK mutant cells (Scheffel et al. 2006). The
process of assembly of MamK filaments is unknown. Pradel et al. (2006) proved the
authenticity of MamK filaments and showed that MamK exhibits linear distribution
inside Magnetospirillum sp. cells even in the area without magnetosomes. The MamK gene
alone is sufficient to direct the synthesis of straight filaments in E. coli, and one extremity
of the MamK filaments is located at the cellular pole. By using dual fluorescent labeling
of MamK, the authors found that MamK nucleates at multiple sites and assembles into
mosaic filaments. Time-lapse experiments revealed that the assembly of MamK filaments
is a highly dynamic and kinetically asymmetrical process. MamK bundles might initiate the formation of a new filament or associate to a preexisting filament. These results
demonstrated the mechanism of biogenesis of prokaryotic cytoskeletal filaments that are
structurally and functionally distinct from the known MreB and ParM filaments. In addition to positioning magnetosomes, other hypothetical functions of the MamK filaments in
magnetotaxis include anchoring of magnetosomes and involving in magnetic reception.
The classical model of polar magnetotaxis predicts that the field-parallel migration
velocity of magnetotactic bacteria increases monotonically with the strength of an
applied magnetic field. Pan et al. (2009) tested this model experimentally on magnetotactic coccoid bacteria that swim along helical trajectories. It turned out that the contribution of the field-parallel migration velocity decreases with increasing field strength
(0.1–1.5 mT). This unexpected observation can be explained and reproduced in a mathematical model under the assumption that the magnetosome chain is inclined with
respect to the flagellar propulsion axis. The magnetic disadvantage, however, became
apparent only in fields stronger than the geomagnetic field, which suggested that magnetotaxis is optimized under geomagnetic field conditions. It is therefore not beneficial
for these bacteria to increase their intracellular magnetic dipole moment beyond the
value needed to overcome Brownian motion in geomagnetic field conditions.
Magnetotactic bacteria benefit from their ability to form cellular magnetic dipoles by
assembling magnetosomes in chains as a means to navigate along geomagnetic field lines
on their way to favorable habitats. Faivre et al. (2010) studied the assembly of magnetosomes
by ferromagnetic resonance spectroscopy using Magnetospirillum gryphiswaldense cultured in a time-resolved experimental setting. The spectroscopic data showed that magnetic particle growth is not synchronized, the increase in particle number is insufficient to
build up cellular magnetic dipoles, and dipoles of assembled magnetosome blocks occur
when the first magnetite particles reach a stable single-domain state. The authors assumed
that these stable single-domain particles can act as magnetic docks to stabilize the remaining and/or newly nucleated superparamagnetic particles in their adjacencies. The authors
postulated that docking is a key mechanism for building the functional cellular magnetic
dipole, which in turn is required for magnetotaxis in bacteria.
3.4.3 Antibacterial Effects
László and Kutasi (2010) investigated the viability of the microbes Saccharomyces cerevisiae, Bacillus circulans, E. coli, Micrococcus luteus, Pseudomonas fluorescens, Salmonella