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M. L. Fdez-Gubieda et al.
7.1 An Introduction to Magnetotactic Bacteria
Magnetotactic bacteria (MTB) are microorganisms able to passively align parallel to
the Earth’s geomagnetic field lines while they actively swim. This behavior, known as
magnetotaxis, is due to the presence of unique intracellular magnetic organelles called
magnetosomes [1–4]. The magnetosomes are intracellular inclusions composed by
a core of magnetic iron mineral, typically magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ),
enclosed by a thin membrane. Both MTB and magnetosomes spark interest among
scientific community due to their special magnetic and structural characteristics
that make them good candidates for nanotechnological applications [5–11]. The
magnetotactic behavior in bacteria was first observed by Salvatore Bellini (1963) [12]
in freshwater samples. He observed bacteria swimming northward persistently, and
then suggested the presence of an internal magnetic compass responsible of the
orientation of the cells. More than ten years later, R. P. Blakemore, in 1975 [13],
observed the magnetic organelles within the bacterial cells and coined the term
magnetosome to refer to them.
Magnetotactic bacteria are aquatic motile microorganisms widespread in freshwater and marine environments [14]. They are easily detected in chemically and redox
stratified sediments and water columns, predominantly at the oxic-anoxic transition zones (OATZ). Bacteria living in OATZ, with vertical chemical gradients, are
continually searching the optimal position in the stratified water column in order
to satisfy their nutritional requirement. Under these circumstances, magnetotaxis is
thought to be a great advantage by increasing the efficiency of chemotaxis [1]. Due
to the inclination, the geomagnetic field lines act as vertical pathways in a stratified environment, therefore the bacteria aligned in the Earth’s field reduce a three
dimensional search to a single dimension, swimming up-downwards the stratified
column (Fig. 7.1). Another possible role of the magnetosomes has been suggested
as detoxifying agents scavenging metal ions or reactive oxygen species [15, 16].
At present, all the MTB described are motile gram-negative bacteria although they
show a great diversity based on the morphology and physiology. The morphotypes
observed, see Fig. 7.2 (top), include curved (a), spirilla (b), cocci (c), rods (d) and
even some colonial bacteria, which form multicellular aggregates [3, 17]. The only
signature trait they share is the ability to swim along the lines of magnetic fields,
including the Earth’s field.
The characteristics of the magnetosomes differ among the different types of magnetotactic bacteria but are consistent within a single species. This fact clearly reflects
that the formation of these biogenic nanoparticles is under strict biological control.
The morphologies of the crystals fit to three main patterns, see Fig. 7.2 (bottom):
cubooctahedral (a), elongated prismatic (b, c) and bullet- or tooth-shaped (d, e).
The size of the magnetic crystals also varies among species, ranging from around
35 to 120 nm. Nevertheless, each species synthesizes magnetosomes with a characteristic size and a narrow size distribution. Interestingly, the diameter range of
the magnetosomes always remains within the range of the room-temperature stable
single-magnetic domain particles [1, 3, 19, 20]. Two different phases can be differ-
M. L. Fdez-Gubieda et al.
7.1 An Introduction to Magnetotactic Bacteria
Magnetotactic bacteria (MTB) are microorganisms able to passively align parallel to
the Earth’s geomagnetic field lines while they actively swim. This behavior, known as
magnetotaxis, is due to the presence of unique intracellular magnetic organelles called
magnetosomes [1–4]. The magnetosomes are intracellular inclusions composed by
a core of magnetic iron mineral, typically magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ),
enclosed by a thin membrane. Both MTB and magnetosomes spark interest among
scientific community due to their special magnetic and structural characteristics
that make them good candidates for nanotechnological applications [5–11]. The
magnetotactic behavior in bacteria was first observed by Salvatore Bellini (1963) [12]
in freshwater samples. He observed bacteria swimming northward persistently, and
then suggested the presence of an internal magnetic compass responsible of the
orientation of the cells. More than ten years later, R. P. Blakemore, in 1975 [13],
observed the magnetic organelles within the bacterial cells and coined the term
magnetosome to refer to them.
Magnetotactic bacteria are aquatic motile microorganisms widespread in freshwater and marine environments [14]. They are easily detected in chemically and redox
stratified sediments and water columns, predominantly at the oxic-anoxic transition zones (OATZ). Bacteria living in OATZ, with vertical chemical gradients, are
continually searching the optimal position in the stratified water column in order
to satisfy their nutritional requirement. Under these circumstances, magnetotaxis is
thought to be a great advantage by increasing the efficiency of chemotaxis [1]. Due
to the inclination, the geomagnetic field lines act as vertical pathways in a stratified environment, therefore the bacteria aligned in the Earth’s field reduce a three
dimensional search to a single dimension, swimming up-downwards the stratified
column (Fig. 7.1). Another possible role of the magnetosomes has been suggested
as detoxifying agents scavenging metal ions or reactive oxygen species [15, 16].
At present, all the MTB described are motile gram-negative bacteria although they
show a great diversity based on the morphology and physiology. The morphotypes
observed, see Fig. 7.2 (top), include curved (a), spirilla (b), cocci (c), rods (d) and
even some colonial bacteria, which form multicellular aggregates [3, 17]. The only
signature trait they share is the ability to swim along the lines of magnetic fields,
including the Earth’s field.
The characteristics of the magnetosomes differ among the different types of magnetotactic bacteria but are consistent within a single species. This fact clearly reflects
that the formation of these biogenic nanoparticles is under strict biological control.
The morphologies of the crystals fit to three main patterns, see Fig. 7.2 (bottom):
cubooctahedral (a), elongated prismatic (b, c) and bullet- or tooth-shaped (d, e).
The size of the magnetic crystals also varies among species, ranging from around
35 to 120 nm. Nevertheless, each species synthesizes magnetosomes with a characteristic size and a narrow size distribution. Interestingly, the diameter range of
the magnetosomes always remains within the range of the room-temperature stable
single-magnetic domain particles [1, 3, 19, 20]. Two different phases can be differ-