Biomedical Engineering Reference
In-Depth Information
production of functional magnetosome nanomaterials. The future roles of mag-
netosomes and the biomimetic synthetic routes to biomedical materials will also
be discussed.
11.4.1
Bacteria Characterization
All of the magnetic bacteria identifi ed to date are Gram-negative, highly motile,
and are located in specifi c regions of chemically stratifi ed aquatic environments,
ranging from marine to fresh-water ponds. Magnetic bacteria are microaerophiles
and are located and isolated from the microaerobic and anoxic region of the oxic-
anoxic transition zone (OATZ) of these environments. These bacteria all have
fl agella (long, rotating fi laments for propulsion) that they use to travel around
these highly chemically defi ned regions.
The fi rst isolated and cultured magnetic bacterium,
Magnetospirillum magneto-
tacticum
MS-1 (Figure 11.3; originally known as
Aquaspirillum magnetotacticum
),
was fi rst isolated from a freshwater pond in Massachusetts. The organism was
found to produce magnetite and to belong to the
- Proteobacteria group. This
bacterium is respiratory, utilizes oxygen and nitrates, and grows chemo- organo-
heterotrophically, using organic acids as carbon and electron sources. Many sub-
sequent magnetic bacteria that have been isolated have shown a large diversity of
morphotypes (Table 11.2); examples include spirillia (helical), bacillus (rod-like),
vibro (curved rod), and cocci (round), though all were of a similar size (1
α
×
1 - 6
μ
m).
The fact that these
-Proteobacteria produced magnetite in a chain motif led to
the assumption of a common evolutionary lineage of the magnetotaxis trait. Mag-
netic bacteria have proved to be notoriously diffi cult to cultivate, most likely
because of their highly defi ned complex and various chemical environments. To
date, although cocci seem to be dominant in environmental samples, the majority
of the cultured strains belong to the
Magnetospirillum
subfamily. Most magnetic
bacteria which have been isolated belong to the
α
- Proteobacteria group, although
several magnetic bacteria have now been identifi ed as deriving from phylogeneti-
cally diverse groups, some of which biomineralize greigite (Fe
3
S
4
) [33 - 35] as
opposed to magnetite; these include multicellular magnetic prokaryote s ( MMP s)
[35-37]. Neither greigite-producing bacteria nor MMPs have yet been cultivated,
and consequently few ecological or genetic data are available to describe them.
However, it has been noted that they are located in sulfi de - rich anaerobic salt - water
environments, and partial DNA sequencing has suggested that one bacterium
belonged to the
α
- Proteobacteria
related to sulfate-reducing bacteria. Originally, it was thought that the composition
of the magnetosomes (greigite or magnetite) was determined by their phylogenetic
group (
γ
-Proteobacteria group, whereas MMPs are
δ
-Proteobacteria), and this led to a proposed theory of
dual evolutionary origins [38]. However, another magnetic sulfate-reducing
α
as opposed to
γ
or
δ
-
proteobacterium has recently been isolated, cultivated and characterized - the
freshwater
Desulfovibrio magneticus
RS - 1 - which, interestingly, biomineralizes
magnetite, in contrast to the other
δ
δ
- Proteobacteria greigite - producers, thus
