How to find magnetic bacteria

The eyes look into a microscope, the hands hold bar magnets close to a drop of mud on the microscope table: students from the Vaterstetten high school are searching for magnetic bacteria.

A strange species: magnetotactic bacteria

The small creatures are correctly called magnetotactic bacteria, i.e. bacteria that orientate themselves on the magnetic field. Each of them allows its own compass to grow within itself: a chain of tiny magnets in the nanometer range. This special arrangement of magnets turns the tiny creatures into sophisticated floating companions and they align themselves along the magnetic field lines. But why do they do that? It is believed that this is how they find their perfect habitat: the top 1-2 cm of mud in bodies of water. After the first representatives of their kind were first described over 50 years ago, a wide range of magnetotactic bacteria have been found in almost all fresh and saltwater environments in recent years.

Magnetotactic bacteria are only about one-twentieth the width of a hair. So what's the best way to track them down? Where do you look for them? And how do you lure them out of their mud under the microscope?

In research laboratories at universities, high-resolution microscopes are equipped with pairs of so-called Helmholtz coils. These current-carrying conductors can generate magnetic fields in any direction and with different strengths. This allows researchers to identify bacteria that respond to a magnetic field. But how can the young researchers at the school observe magnetotactic bacteria without a well-equipped laboratory?

As part of the Humboldt Academy for Science and Engineering (HASE for short) at the Humboldt-Gymnasium Vaterstetten, a group of interested and committed eighth to tenth graders (HASEs) looked for suitable tools in their school: microscopes from the biology collection, bar and horseshoe magnets from the physics collection . They took small drops from mud samples and placed them under the microscope. A bar magnet with a south pole facing the water drop causes the magnetotactic bacteria to swim straight towards the edge of the water drop. At least in theory. Because when the students used the same procedure in their school, they saw - nothing.

Detective work

What could that be about? Was the magnetic field of the bar magnet not strong enough? No, the students measured the magnetic field at various distances from the bar magnet and it should be more than sufficient. Was the resolution of the school microscope not high enough? No, here too the students were in contact with the researchers and used similar lenses with the same magnification. Were there no magnetotactic bacteria in the mud sample? Actually no, they had received the mud samples with magnetotactic bacteria in advance from researchers at the LMU. Had the bacteria possibly died in the meantime? All tips for storing the samples were followed exactly.

Finally, the HASEs packed up their school microscope and the magnets they used and drove to the LMU in order to be able to directly compare their procedure with that of the researchers.

It quickly became clear why the first observation attempts had failed: While the researchers' inverted microscope looked at the drop through the glass plate from below, the students looked at the drop from above with their microscopes and were, so to speak, blinded by the scattering on the water surface. The light from the microscope reflects back through the surface of the water, preventing students from seeing into the drop. With a little trick, the magnetotactic bacteria can also be observed with a school microscope: With a courageous swing, the glass plate with the drop of mud on it is simply turned over and the drop now hangs at the bottom of the glass plate. The mud slowly sinks to the deepest part of the drop, the edges clear up, and the magnetotactic bacteria can now be lured there using a simple bar magnet in order to observe them. Mission successful.

Dancing bacteria - and other strange shapes

The students used their morning at the LMU laboratory to observe the small animals using the researchers' professional microscope. The Helmholtz coils on this microscope offer the possibility of making the magnetotactic bacteria dance: depending on the direction in which the magnetic field is applied, the animals follow the field lines without will. They can even be made to swim in circles - much to the amusement of the students. On a trip into the interior of the mud sample, the wondrous zoo of non-magnetic microbiology was revealed: bell animals that contract suddenly and then slowly unfold again to swirl water into their throats, animals that move so quickly that they can hardly keep up with it Microscope can be followed, and small jellyfish-like structures, which repeatedly crossed the observation room.

Making plans

The students are already pursuing their next plans: They want to show their classmates and exchange students from Estonia how they can find magnetotactic bacteria using equipment from their school. To do this, they write instructions in English and record videos in which they demonstrate the preparation of the mud drop on the microscope slide. In the longer term, they plan to attach the Helmholtz coil from the physics collection or self-wound coils to the biology microscope and thus make observation more comfortable.

We are already looking forward to your next visit!