In microscopy, researchers find observing dead, stationary specimens easier than observing living, motile specimens such as bacteria. This difficulty arises from the need for researchers to anchor living specimen within the aperture of the microscope—limited field of vision, an issue recently and successfully addressed by researchers concerning the manipulation of magnetic bacteria (1).
Magnetotactic bacteria, often referred to as living compass needles, are among few creatures that can detect and interact with magnetic fields. This ability stems from the presence of chains of organelles, called magnetosomes. Magnetosomes consist of vesicles containing submicroscopic magnetic particles. By detecting the torques produced by magnetic fields in these chains, magnetotactic bacteria can align themselves along these magnetic fields (1, 2).
Scientists have previously attempted, with varying degrees of success, to exploit the susceptibility of magnetotactic bacteria to magnetic fields in order to manipulate them under the microscope (1). Attempts have ranged from wrapping microscopes with electric coils to more complex instruments and fixtures, both of which were cost-ineffective and impractical for field studies (1). Furthermore, inconsistencies in the magnetic fields produced by these instruments often caused specimens to drift out of the focal plane, or area of visibility (1, 2).
Magnetotactic bacteria, often referred to as living compass needles, are among few creatures that can detect and interact with magnetic fields. (Source: Flickr)
However, a joint research effort led by Smid et al. has recently developed a new tool capable of manipulating magnetotactic bacteria while compensating for the drift of specimens by using a rotating permanent magnet. Their design, reminiscent of a clock, consists of a series of gears that rotate a disk. Atop the disk sits a permanent magnet, which generates its own magnetic field (1, 2). To test the apparatus’s effectiveness, the research team conducted observational studies of bacteria under the influence of a rotating magnetic field of near constant intensity but varying frequencies. They also derived mathematical models for the movement of the bacteria affected by the field in order to compare the tool’s theoretical performance to the experimental observations performed later (1).
According to AIP Reviews of Scientific Instruments, the relatively simple design of a rotating permanent magnet produced a magnetic field of more consistent intensity than did previous attempts at cell manipulation with Helmholtz coils (1, 2). [MK1] Furthermore, they found that timing the rotation of the magnet such that the magnet was closer to the specimen when its field was weakest minimized drift and allowed observers to maintain specimen within the focal plane for far longer periods of time. Finally, the team discovered that while the bacteria were capable of aligning and propelling themselves synchronously along the rotating magnetic field, they would eventually begin to asynchronously follow the field once rotation reached an “escape frequency”. This highlights the limitations of magnetic and hydrodynamic properties of magnetotactic bacteria (1).
Ultimately, this advancement in cell manipulation has opened new avenues not only for economically studying magnetotactic bacteria within the laboratory, but also for conducting observations of these creatures within their natural habitats. According to Smid, this technique will allow scientists to begin focusing not only on the diverse range of responses of different species of magnetactic bacteria to varying magnetic fields, but also on the practical benefits of their magnetic properties (2).
References:
- Pieter Smid, Valeriy Shcherbakov, Nikolai Petersen. Microscopic observation of magnetic bacteria in the magnetic field of a rotating permanent magnet. Review of Scientific Instruments, 2015; 86 (9): 095106 DOI: 10.1063/1.4929331
- American Institute of Physics. (2015, September 15). New tool for studying magnetic, self-propelled bacteria that resemble compass needles. ScienceDaily. Retrieved September 18, 2015 from www. Sciencedaily.com/releases/2015/09/150915135407.html