anaerobic and microaerobic conditions, whereas the final maturation of GFP
requires oxygen, growth parameters had to be optimized to perform both tasks. It
was found that the most abundant magnetosome membrane protein, MamC, which
is the ortholog to Mms13, serves best as anchor protein for possible fusions (Lang
and Sch€ uler 2008).
As proofs-of-principle, several biotechnologically relevant assays based on
magnetosomes as magnetic carrier colloids have been developed, such as
immunoassays for pollutants, hormones and toxic detergents, ligand-receptor binding assays, and target cell separation methods (Tanaka and Matsunaga 2000;
Matsunaga et al. 2003; Kuhara et al. 2004; Yoshino et al. 2004). Furthermore,
modified magnetosomes have been used to extract DNA and to discriminate singlenucleotide polymorphisms by automated systems (Tanaka et al. 2003; Yoza et al.
2003).
More recently, magnetosomes have been tested as potential drug carriers for
antitumor treatments (Sun et al. 2007; Sun et al. 2008) and as a contrast agents for
MRI (Lisy et al. 2007). Particularly for the latter application, magnetosomes are
becoming interesting, because magnetic single-domain particles with anisotropical
shapes elongated along an easy magnetization axis show longer relaxation times
which results in higher imaging contrast (Vereda et al. 2009).
1.5 Conclusion and Outlook
In this chapter, we described magnetotactic bacteria and their remarkable capability
to biomineralize magnetite nanoparticles, so-called magnetosomes. This material is
extraordinary because it has been optimized with respect to its function as a
magnetic field actuator. The crystallites are made of stoichiometric magnetite,
which is the most efficient iron oxide phase with respect to magnetization and
iron usage. Particles are in the single-magnetic-domain size range and this feature is
optimal for their function. They are sometimes elongated along the easy magnetization axis, improving the stability of the magnetic dipole against thermal
fluctuations. Finally, chain formation of magnetosomes adds magnetic dipoles of
single particles and enables the bacteria to align along the field lines of Earth’s
magnetic field.
Many aspects of this biomineralization process remain unclear. Iron uptake and
particularly transport into the intracellular vesicles has still to be resolved. Furthermore, it is not understood how MTB are able to form the appropriate iron phase
under the very soft physiological chemical conditions. Compartmentalization might
allow them to control local pH and redox potential in order to precipitate the
magnetite phase. However, it has not been shown which conditions exist within
the magnetosome organelles. Proteins that are involved in all the stages of magnetite formation and particle chain alignment have been identified to some extent, but
their specific roles remain widely unclear. Concerning the chemistry, one particularly interesting fact is that MTB are able to produce particle morphologies that
1 Magnetite Biomineralization in Bacteria
21
requires oxygen, growth parameters had to be optimized to perform both tasks. It
was found that the most abundant magnetosome membrane protein, MamC, which
is the ortholog to Mms13, serves best as anchor protein for possible fusions (Lang
and Sch€ uler 2008).
As proofs-of-principle, several biotechnologically relevant assays based on
magnetosomes as magnetic carrier colloids have been developed, such as
immunoassays for pollutants, hormones and toxic detergents, ligand-receptor binding assays, and target cell separation methods (Tanaka and Matsunaga 2000;
Matsunaga et al. 2003; Kuhara et al. 2004; Yoshino et al. 2004). Furthermore,
modified magnetosomes have been used to extract DNA and to discriminate singlenucleotide polymorphisms by automated systems (Tanaka et al. 2003; Yoza et al.
2003).
More recently, magnetosomes have been tested as potential drug carriers for
antitumor treatments (Sun et al. 2007; Sun et al. 2008) and as a contrast agents for
MRI (Lisy et al. 2007). Particularly for the latter application, magnetosomes are
becoming interesting, because magnetic single-domain particles with anisotropical
shapes elongated along an easy magnetization axis show longer relaxation times
which results in higher imaging contrast (Vereda et al. 2009).
1.5 Conclusion and Outlook
In this chapter, we described magnetotactic bacteria and their remarkable capability
to biomineralize magnetite nanoparticles, so-called magnetosomes. This material is
extraordinary because it has been optimized with respect to its function as a
magnetic field actuator. The crystallites are made of stoichiometric magnetite,
which is the most efficient iron oxide phase with respect to magnetization and
iron usage. Particles are in the single-magnetic-domain size range and this feature is
optimal for their function. They are sometimes elongated along the easy magnetization axis, improving the stability of the magnetic dipole against thermal
fluctuations. Finally, chain formation of magnetosomes adds magnetic dipoles of
single particles and enables the bacteria to align along the field lines of Earth’s
magnetic field.
Many aspects of this biomineralization process remain unclear. Iron uptake and
particularly transport into the intracellular vesicles has still to be resolved. Furthermore, it is not understood how MTB are able to form the appropriate iron phase
under the very soft physiological chemical conditions. Compartmentalization might
allow them to control local pH and redox potential in order to precipitate the
magnetite phase. However, it has not been shown which conditions exist within
the magnetosome organelles. Proteins that are involved in all the stages of magnetite formation and particle chain alignment have been identified to some extent, but
their specific roles remain widely unclear. Concerning the chemistry, one particularly interesting fact is that MTB are able to produce particle morphologies that
1 Magnetite Biomineralization in Bacteria
21