Biomedical Engineering Reference
In-Depth Information
although single-domain particles may be formed under certain conditions [26].
The
in vitro
formation of magnetite may also be catalyzed by bacteria or fungi [27].
Equally, magnetite nanoparticles can be biomineralized intracellularly, and these
particles are present in many higher organisms including honeybees, pigeons,
pelagic fi sh, bats, rodents, and humans [28]; typically, the human brain contains
approximately 5
1 0
6
crystals per gram of tissue. Although the purpose of these
particles has not yet been assigned in all cases, the majority are proposed to have
sensing/navigational roles. They have also been considered to have a destructive
role when present in excess, with recent studies having been conducted to inves-
tigate possible associations between increased magnetite levels in the human brain
and neurodegenerative conditions such as Alzheimer' s and Parkinson ' s diseases
[29] .
Finally, the occurrence of intracellular magnetite in prokaryotes is limited to few
bacteria, all of which are magnetic. These bacteria deliberately biomineralize
highly morphologically defi ned single-domain crystals of magnetite within their
cells; it is this nanomaterial that will be considered for nanomagnetic medical
applications.
×
11.4
Magnetosomes: Biomineralization in Magnetic Bacteria
Although magnetotactic bacteria were fi rst observed by Bellini in 1963 [30], they
were not widely reported until 1975, following their (independent) rediscovery by
Blakemore [31]. Both Bellini and Blakemore observed that some bacteria in envi-
ronmental samples consistently moved north, and would respond to the move-
ment of a magnet. Magnetic bacteria are ubiquitous and are found in a wide variety
of shapes, sizes, and environments; what unites them is their magnetotaxis - the
ability to recognize and align with a magnetic fi eld. This property is due to the
presence of single-domain, magnetic iron mineral nanoparticles (magnetite, but
occasionally greigite, Fe
3
S
4
) within the bacterial cell. These nanomagnets, which
are biomineralized within lipid vesicles, are known as magnetosomes, and are
usually arranged in chains of 20-30 magnetosomes in length, lying approximately
parallel to the principal axis of the cell (Figure 11.3). The motifs, chain arrange-
ments and number of chains may vary from strain to strain, however.
Magnetic bacteria and magnetosomes have attracted a huge amount of research
interest since the late 1970s. While the initial research investigations concentrated
on the analysis of the magnetite mineral, most of the subsequent studies focused
on the biomineralization processes from a biological perspective.
Magnetic bacteria are the smallest and simplest organisms capable of perform-
ing biologically controlled mineralization within their cells and, as such, provide
an ideal model to study and help understand the biomineralization process. This
is of inherent importance for both practical and academic reasons. In practical
terms, a better understanding of biomineralization will help to further medicine
and improve biomaterials such as artifi cial bones, as well as open new possibilities
