Living magnets: Magnetotactic bacteria in the environment and their applications for environmental remediation and human medicine

Our world is filled with a vast array of microorganisms with strange and fascinating abilities. One example is magnetotactic bacteria, a group of organisms that respond to magnetic fields. Put a magnet near them, and they'll come swarming toward it!

Scientists are excited by the potential applications of microbes that can be moved around in targeted ways. They are exploring several biotechnology applications, from cancer treatment to metal recycling. To understand how scientists are harnessing the power of magnetotactic bacteria, we must first understand the mechanics of their unique ability.

Magnetotactic bacteria: nature's magnets

Magnetotactic bacteria (MTB) are sediment-dwelling organisms that navigate their environment using Earth's magnetic field. This movement is called magnetotaxis. It is facilitated by magnetosomes, small crystals of magnetite (Fe₃O₄) or greigite (Fe₃S₄), which align in chains through the center of the cell. These tiny cellular magnets help MTB align north or south with Earth's magnetic field, like the needle of a compass.

MTB use magnetotaxis to find areas of optimal oxygen concentrations in their oceanic and sediment habitats. They thrive in areas where oxygen is low but not totally absent, called microaerophilic zones. By aligning with Earth's magnetic field, magnetotactic bacteria simplify the process of moving through their environment. Instead of moving in 3 dimensions, they can move in one dimension along magnetic field lines until they reach microaerophilc zones.

In the environment, magnetotactic bacteria are important cyclers of iron and other metals. Magnetosomes are mostly iron by weight, so MTB are constantly synthesizing and recycling the metal. They also take up manganese and incorporate it into magnetosomes beside the magnetite. They accumulate other metals, like selenium and tellurium, into separate granules. And like many organisms, MTB can bind metals to their cell surface. These interactions form an important part of the metals' biogeochemical cycles, especially in marine environments, where iron is a limiting nutrient and magnetotactic bacteria can make up almost 30% of the total microbial population.

The genes for magnetotaxis are found in 16 distinct microbial lineages. Organisms with this capability include nitrogen cyclers, photosynthetic groups, and a vast array of organic matter metabolisms. But interestingly, magnetotaxis is a monophyletic trait, meaning that the genes evolved only once in earth's history. This means that all microbial groups with magnetotaxis likely share a common ancestor, even though they have few other characteristics in common.

Harnessing magnetotactic bacteria for human applications

Medical uses of magnetotactic bacteria take advantage of the natural response of MTB to magnetic fields. When a magnet is placed near MTB, they will move toward it. This means that if doctors place a magnet on a tumor, they can deliver MTB, or MTB-derived nanoparticles, directly to the cancer cells. Magnetotactic treatments currently in use include thermal treatment that physically breaks down the cancer cells, and targeted drug delivery directly to the effected area.

Magnetotactic organisms can also be harnessed to protect the environment from heavy metals. This includes properly recycling electronic wastes to prevent pollution, and remediating metal pollution from the environment. As previously mentioned, MTB can take up manganese, selenium, and tellurium into their cells. They also accumulate silver, gold, copper, cobalt, and chromium on the cell surface. MTB will naturally collect these metals in any environment, including a polluted ecosystem or a solution of ground-up eWaste. A magnet can then be used to harvest the cells, recovering the metals for reuse in future electronics.

Magnetotactic bacteria may also increase the metal uptake capacity of plants used for heavy metal bioremediation. When E. coli modified with magnetic nanoparticles colonized plant roots, they increased the plants' capacity for metal uptake. They also led to the assembly of a more diverse microbial community on the plant roots, which protected the plant from damage by high levels of metals.

Finally, MTB can serve as a model for studying the evolutions of multicellular life. Small multicellular organisms called multicellular magnetic prokaryotes produce chains of magnetosomes that stretch across multiple cells. They may represent a stage somewhere between individual cells that produce magnetosomes, and multicellular organisms with specialized magnetotactic cells or organs.

The future of magnetotactic bacteria

Magnetotactic bacteria are important environmental metal cyclers with promising applications for human and environmental health. There are several challenges in the widespread use of MTB-based biotechnologies, including the difficulty in obtaining cell yields and potential negative correlation between metal uptake and magnetosome formation. Both challenges are being addressed in ongoing MTB research around the world.

In my opinion, the most exciting application of magnetotactic bacteria is in the recycling of eWaste! We currently produce millions of tons of eWaste every year, and only a fraction of it is properly recycled. If we can use MTB to harvest metals from old technology and recycle them into new products, we will significantly reduce the metal pollutant burden on the environment. We will also decrease our reliance on exploitative heavy metal mining practices.

If you'd like to learn more about magnetotactic bacteria, check out my Essentially Microbial Clubhouse Chat Magnetotactic Bacteria: from environment to medicine. If you want to know how microbes can help you with your current project, reach out to AppliedMicrobio today!