Magnetotactic Bacteria

Introduction to Magnetotactic Bacteria

Figure 1 - Model of magneto-aeotaxis. Magnetotactic bacteria (black) use the earth's magnetic field (gray lines) as guides to localize to the OATZ while other organisms (white) must rely on other methods for finding the same region. Courtesy of Komeili (http://femsre.oxfordjournals.org/content/36/1/232.figures-only)

Figure 2- Electron micrograph of Magnetospirillum gryphiswaldense cells containing chains of magnetite crystals (top) and magnified section of crystal chain (bottom). By Caulobacter subvibrioides (Diskussion) GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Figure 3- Proposed model of magnetosome formation. "First, the reshaping of the inner cell membrane by MamI, MamL, MamQ, MamB, and other factors (green) creates the magnetosome membrane. Second, MamE is recruited to the nascent magnetosome. Third, MamE (in blue), independent of its protease activity recruits other proteins (red) to the magnetosome. Fourth, MamK and MamJ help to organize magnetosomes into chains. Note that this step is independent of biomineralization and may occur before or after crystal formation. Fifth, biomineralization is initiated, and small crystals of magnetite are formed. Finally, these small crystals are matured into large crystals in a step that requires the proteolytic activity of MamE." Komeili (2012) Courtesy of Komeili (http://femsre.oxfordjournals.org/content/36/1/232.figures-only)

Magnetotactic bacteria (MB) are gram-negative bacteria that build specialized organelles called magnetosomes in order to store magnetic material and align themselves with the earth’s magnetic field. Magnetotactic bacteria were first described in 1975 when Richard Blakemore realized that a specific group of bacteria he collected from sediment constantly swam in the same geographic direction, regardless of the positioning of the microscope or external stimuli ^[1]. MB are mostly found in shallow aquatic environments where oxygen and other redox compounds are horizontally stratified and many described magnetotactic bacteria localize at or close to the oxic anoxic transition zone (OATZ)—a region in the water column that has very low oxygen levels ^[2]. The current model (shown in Figure 1) to explain the selective advantage provided by magnetosomes is that magnetotactic bacteria are able to locate the OATZ much easier than bacteria that solely use chemotactic and aerotactic mechanisms ^[3]. Although the magneto-aerotaxis model has been widely accepted amongst the scientific community, new research is suggesting that the behavior magnetotactic bacteria exhibit in the environment may be more complicated than a simple response to oxygen levels:

Some MB species also show phototactic response, which helps reinforce magneto-aerotactic behavior and repel them from surface waters ^{[4, 5]}
Genome sequences show that MB have some of the highest numbers of signaling proteins of Bacteria ^[6]
MB produce more magnetosomes than necessary to align with the earth’s magnetic field ^[7]
MB have been found near the equator, where their magneto-aerotactic behavior has no advantage ^[8]

Regardless of the actual biological function of magnetosomes, magnetotactic bacteria are a very interesting research topic because they have the potential to impact a large number of scientific and applied disciplines. Magnetosomes are the perfect model for the study of cellular compartmentalization and organization in bacteria because the formation of specialized, membrane bound, features is usually attributed to eukaryotic organisms. Magnetotactic bacteria are also a great model for the study of biomineralization because of the precise control over the composition, size and morphology of magenetite crystals in magnetosomes.

Magnetosome Island

Identification of the genetic elements needed for magnetosome formation took many years due the lack of cultured and genetically tractable model organisms. Aquaspirillum magnetotacticum, now referred to as Magnetospirillum magnetotacticum, or MS-1, was the first magnetotactic bacteria to be isolated in pure culture. Two closely related species, Magnetospirillum grysphiswaldense(MSR-1) and Magnetospirillum magneticum(AMB-1), were soon isolated and have since become the focus of most magnetosome research^9,10.