Magnetotaxis: A Bacterial Superpower

The idea of bacterial superpowers is perhaps most associated with superbugs: the terrifying, drug-resistant bacterial strains that appear ever more frequently in news reports. While the notion of a world where antibiotics no longer work is chilling, this blog post will focus on a more positive aspect of the bacterial domain.

One of the more “niche” bacterial superpowers is magnetotaxis: the ability of certain bacteria to align their motion to the Earth’s magnetic field. This phenomenon was first reported in 1963 by Salvatore Bellini in the University of Pavia. While observing bog sediment under the microscope, he noticed a set of bacteria orienting themselves in the same direction: towards the Earth’s magnetic North pole. He dubbed these gram-negative bacteria “magnetosensitive”, or “batteri magnetosensibili”, but the discovery went largely unnoticed by the international scientific community [1]. The name “magnetotactic bacteria” (MTB) was introduced about a decade later, when Richard Blakemore reported the same phenomenon for bacteria found in marine sediments [2]. Through transmission electron microscopy, Blakemore was also able to capture the cellular feature that gives MTBs their unusual abilities: a rod-like structure of membrane-bound, iron-rich inorganic crystals, called magnetosomes. Later it was revealed that this structure is supported by a dedicated cytoskeletal system, which keeps it rod-shaped and prevents the aggregation of magnetosomes [4]. Magnetotaxis then results from the combination of the passive alignment of the cell to the Earth’s magnetic field, and flagellar motion.

There are two main groups of MTBs, differentiated by the type of mineral contained in their magnetosomes: magnetite (Fe3O4) or greigite (Fe3S4). These two groups are now believed to have evolved independently from each other [3]. It is thought that magnetotaxis confers an evolutionary advantage by allowing bacteria to maintain their direction and find optimal growth conditions along chemical and redox gradients [4]. Many of these bacteria live on the boundary between oxygen-rich and oxygen poor sediment, but require low oxygen concentrations for optimum growth. Magnetotaxis then restricts their search along one dimension, allowing them to find their preferred conditions more easily.

So why should we care about these bacteria? Apart from the inherent “coolness” of a bacterium that can sense the Earth’s magnetic field, these organisms are also important for their potential use in a wide variety of applications, ranging from nanomaterials to paleo-climatology. Magnetite magnetofossils dating from as far back as 700 million years ago have been reported, and can reveal information about fluctuations in the Earth’s magnetic field in past ages [6]. Magnetosomes have been used as carriers for various macromolecules (enzymes, nucleic acids, and antibodies), as the surface of the magnetosome membrane has proteins and lipids with exposed functional groups, to which macromolecules of interest can be covalently coupled. The magnetic nature of the magnetosome is then used to isolate these macromolecules, or guide their delivery [7].

Overall, despite their name’s connotation to the villain in a popular superhero franchise, magnetotactic bacteria are not very threatening, and their unique bacterial superpower can be harnessed for good.

References:

  1. Bellini, S. (1963). Ulteriori studi sui “ Batteri Magnetosensibili ”. Institute of Microbiology, University of Pavia, Italy.
  2. Blakemore, R. (1975). Magnetotacic Bacteria. Science, 190(4212), 377–379.
  3. DeLong, E. F., Frankel, R. B., & Bazylinski, D. A. (1993). Multiple evolutionary origins of magnetotaxis in bacteria. Science, 259(5096), 803–806. http://doi.org/10.1126/science.259.5096.803
  4. Lefèvre, C. T., Abreu, F., Lins, U., & Bazylinski, D. A. (2011). A Bacterial Backbone: Magnetosomes in Magnetotactic Bacteria. In M. Rai & N. Duran (Eds.), Metal Nanoparticles in Microbiology (pp. 75–102). Berlin, Heidelberg: Springer Berlin Heidelberg. http://doi.org/10.1007/978-3-642-18312-6_4
  5. Philippe, N., & Wu, L. F. (2010). An MCP-Like Protein Interacts with the MamK Cytoskeleton and Is Involved in Magnetotaxis in Magnetospirillum magneticum AMB-1. Journal of Molecular Biology, 400(3), 309–322. http://doi.org/10.1016/j.jmb.2010.05.011
  6. Yan, L., Zhang, S., Chen, P., Liu, H., Yin, H., & Li, H. (2012). Magnetotactic bacteria, magnetosomes and their application. Microbiological Research, 167(9), 507–519. http://doi.org/10.1016/j.micres.2012.04.002
  7. Sun, J. B., Duan, J. H., Dai, S. L., Ren, J., Guo, L., Jiang, W., & Li, Y. (2008). Preparation and anti-tumor efficiency evaluation of doxorubicin-loaded bacterial magnetosomes: Magnetic nanoparticles as drug carriers isolated from Magnetospirillum gryphiswaldense. Biotechnology and Bioengineering, 101(6), 1313–1320. http://doi.org/10.1002/bit.22011

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