Living magnets

Bacterial responses to environmental aspects are effective. Certain bacteria has got magnetism property and hence referred as living magnets. These bacteria can sense the magnetic field and make their movement according to earth’s magnetic field. They are referred as magnetotactic bacteria. Sensing ability according environment magnetic field is referred as magnetotaxis. It was first identified in aquatic bacteria by Scientist R.P. Blakemore in the year 1975. The aquatic magnetotactic bacteria make their orientation accordingly. Even when these organisms are not alive they can orient themselves as magnetic compass needle does.

Magnetotactic bacteria

A Magnetotactic bacterium is a type of bacteria which was first introduced in late 1960’s.  These bacteria have special organelles called magnetosomes. Magnetosomes has some magnetic crystals. When viewed under microscope they start orienting in particular direction. It is because these bacteria precipitate either magnetic particle with in their cells. Some of which kept for record are referred as magneto fossils.

Bacterial magnetic particles

Shape, size, layout of each magnetotactic bacteria is different. They can excrete in two forms.

• Magnetite
• Greigite

Magnetite has magnetic characteristics three fold more than greigite. These bacteria can survive in limited oxygen environment. Transition zone of limited oxygen and oxygen rich zone results in growth of magnetotactic bacteria which produce magnetite. Transition zone is referred as oxic- anoxic transition zone. With the help of electron acceptors like nitrate, sulfate and nitric acid magnetotactic bacteria is produced under anaerobic conditions. When atmospheric oxygen concentration increases dissolved iron concentration decreases. Ultimately organisms absorb this iron in desired forms and later utilize to form magnetosomes for magnetotaxis. Magnetic particles are arranged in chains in magnetotactic bacteria. They are characterized by gram negative bacterial cell wall. With the help of flagella, magnetotactic bacteria can move. Flagella are arranged in polar or bipolar form.

Development of magnetic crystal

Magnetic crystal is driven by below mentioned two factors.

• Align magnetic force with magnetic crystal
• Reduction crystal’s magnetic force

These forces cause a magnetic domain. Cells align according to the polarity of either magnetotaxis or aerotaxis. Bacteria move towards optimal oxygen concentration by aerotaxis property coupled with magnetotactic bacteria. Unidirectional bacteria swimming can be reversed back when there is very low optimal oxygen concentration. They can swim opposite to oxic conditions when there is very high optimal oxygen concentration. This behavior when observed in bacterial cells is referred as magneto aerotaxis.  Magneto aerotactic has two types of mechanism.

• Polar magneto aerotactic
• Axial magneto aerotactic

When the bacterial cells move in only one direction along the earth’s magnetic field it is polar magneto aerotactic for case in point magnetotactic cocci. When the bacterial cells move in both the directions it is axial magneto aerotactic.

Magnetosomes

Iron absorbed by the bacteria is immediately converted to magnetite by magnetosomes forms magnetic crystals and there is no accumulation of any intermediate iron compounds. This confirms that Biomineralisation property already exists inside the bacteria. Biomineralisation is a biological property of magnetotactic bacteria which are controlled by following factors.

• Iron concentration
• Nucleation of crystal
• Redox potential
• pH

Iron is not replaced by any other transition metals like titanium, mercury, lead, copper, cobalt, chromium or nickel in the magnetotactic bacteria Magnetospirillum magnetotacticum. Magnetospirillum magnetotacticum is aquatic bacteria is arranged in the form of chains. It has membrane bound organelles that exist as magnetite in Fe₃O­­­₄ ­­form. Marine bacteria move downwards facing towards sediments by magnetic orientation force. These organisms are found plenty in equatorial zone. In equatorial zone, magnetic field acts in horizontal direction. Greigite is precipitated by some extremophile bacteria isolated in sulfurous environments in the form of Fe₃S­­­₄ iron sulfide compound.

Biological applications of living magnets

It helps in interaction of bioactive compounds. Magnetotactic bacteria help in easy prediction of South Pole of meteorites and rocks. Living magnets helps in detailed study of magnetic domain analysis. Other applications of living magnets are as follows.

• Enzyme immobilization
• Magnetic antibody formation
• Immunoglobin G (IgG) quantification
• Detection of E.Coli
• Removal of E.Coli
• Introduce gene in to bacterial cells

Magnetosomes are found in bird’s head, green turtles, dolphins, tuna and other higher class organisms akin to animals for assisting their motility or orientation direction.