Researchers at Aalto University in Finland have found a way to use magnets to line up bacteria while they swim. This approach provides more than just a way to order bacteria – it also provides a useful tool for a wide range of research, such as work on complex materials, phase transitions and condensed matter physics.
Magnetic nanoparticles are mixed with a dense bacterial suspension. Switching a uniform magnetic field on and off causes the system to transition between long-range orientational order and active turbulence. Image credit: Kazusa Beppu/Aalto University
Bacterial cells are generally not magnetic, so magnets do not directly contact the bacteria. Instead, the bacteria are mixed in a liquid with millions of magnetic nanoparticles. This means that the rod-shaped bacteria effectively have non-magnetic spaces inside the magnetic fluid. When the magnets are turned on, a magnetic field is created, prompting the bacteria to line up along the magnetic field because any other arrangement takes more energy – the rod-shaped pores are forced to line up with the magnetic field. It is difficult to keep it at an angle.
‘We have managed to completely control the regular Bacillus subtilis Bacteria with magnetic fields. Unlike some rare magnetotactic bacteria, these bacteria are not magnetic, says assistant professor Jaakko Timonen who led the study,
‘In short, the most stable arrangement is to align the bacteria’s “pores” with the magnetic field, resulting in a torque on the bacteria’s body, which pushes them into line,’ explains the postdoctoral researcher. Kazusa Beppu,
The strength of the magnetic field controls the alignment of the bacteria. When the magnets were turned off, the bacteria started floating around randomly. As the researchers dialed up the strength of the magnetic field, the bacteria aligned more and more, eventually swimming in almost perfect rows.
There was also a difference in the amount of bacteria. When population density was high, a stronger magnetic field was required to bring the bacteria into alignment. This is because floating bacteria affect the fluid in a similar way to turbulence. When there are a lot of bacteria, the turbulence-like effect is stronger, and a stronger magnetic field is required to overcome it.
‘Fluid flow created by bacteria in dense suspension is called active turbulence because it contains structures characteristic of turbulent flow such as vortices. However, it is important to understand that this so-called active turbulence is fundamentally different from the normal turbulence encountered in aviation, for example,” Timonen explains.
Active disturbance is an extremely common phenomenon in nature. It is caused by the combined actions of individual units, such as cells that are swimming or moving, i.e. Bacteria, sperm or epithelial cells. ‘Active turbulence is an important research topic in active matter physics, and the dense bacterial suspensions in our system are an excellent tool to study it,’ says Beppu.
The micro-courier of the future?
Ultimately, as fun as it sounds, this work isn’t just about making bacteria swim in an orderly manner. The ability to control the motion of bacteria along turbulent flows is key to understanding and manipulating active substances – materials in which dynamic patterns emerge from the behavior of individual parts. Think of a flock of birds, but on a cellular level.
Researchers envision applications around self-sustaining materials or harnessing the potential of microrobotics or biological engines to harvest energy or transport materials. For example, targeted drug delivery can occur at the micro scale.
‘It is exciting to be able to spatially and versatilely control an active substance over a space much larger than the size of individual active units,’ says Beppu. ‘And because our method is versatile, it can be applied not only to bacterial systems but also to many other systems, which will greatly advance the experimental study of the active substance.’
The ability to fine-tune alignment in this way will also be an invaluable tool in other research domains, such as work on phase transitions or condensed matter physics. In the meantime, the researchers plan to expand their work by testing what happens when the magnetic field is dynamic – for example, with a rotating magnetic field. Beppu expects to see a rich variety of new phenomena in those experiments, adding that ‘understanding the magnetic controllability of orientation and flow is important for designing functional active materials.’
Source: Aalto University
