Could magnetic bacteria be the next generation of microbots?

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The cutting edge of robotics may not be a smarter Siri or a less-creepy humanoid Japanese robot. It might be a swarm of bacteria, compelled to do our bidding through a remotely controlled magnetic field.

Some of the biggest technological advances of the past two decades have involved scaling things down. The development and continual improvement of microprocessors has revolutionized home computing. And the emerging field of nanomedicine promises to transform biomedical science in many ways: from targeted drug delivery to tissue repair at the cellular level.

However, working at the microscale (thousandths of a millimeter) or nanoscale (millionths of a millimeter) poses major challenges. To create a circuit out of microscopic components, we need very small tools that can do the work for us. We need microscopic robots, or microbots.

Artist's depiction of a nanobot performing cell surgery. Credit:

Artist’s depiction of a nanobot performing cell surgery. Source: yalescientific.org 

But building useful microbots is no easy task. Power supply has always been one of the toughest challenges. Some bots contain a very lightweight battery. Others possess a coin cell that scavenges vibrational or light energy from its surroundings. Telling our microbots where to go is another hurdle. Microbots typically don’t work alone- we usually need a swarm of them to perform a task. And we’d like to be able to direct that swarm with the utmost precision.

Some scientists think the solution to our microbot woes can be found in nature. Bacteria are essentially organic “machines” that use light or chemical energy to move about and do work. They fit the size criteria, and they’re everywhere. Rather than re-invent the wheel, what if we could train bacteria to do work for us?

To answer this question, microroboticists are now looking to a group of bacteria that possesses an astounding property: magnetism.

So-called magnetotactic bacteria are promising because their motion can be guided by an externally applied magnetic field. Given the right magnetic field, we might be able to coordinate the motion of thousands to millions of magnetotactic bacteria at once.

Source: iGEM2009

Source: iGEM2009

Magnetotactic bacteria are a genetically diverse group of organisms thought to have evolved during the early Proterozoic, some 2.5 billion years ago, when rising atmospheric oxygen concentrations were reducing the amount of dissolved iron in Earth’s oceans. It’s believed this caused some bacteria to start stockpiling iron. Eventually, these iron stores were adapted to form magnetosomes, crystalline structures found in the cell membranes of modern magnetotactic bacteria.  Magnetosomes align in a chain, allowing the bacterium to orient itself like a compass needle to the local magnetic field.

The most obvious advantage to a magnetotactic microbot is that its motion can be guided by a user-generated magnetic field. But bacteria are also advantageous because they possess flagella. These rotating, tail-like structures act as a propeller, allowing bacteria to swim about quickly and change direction with ease. And the energy required to turn a flagellum is generated by the bacterium’s own metabolism. Power source, check. Motor, check.

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface Electron micrograph of H. pylori possessing multiple flagella (negative staining). Credit: Wikipedia

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface Electron micrograph of H. pylori possessing multiple flagella (negative staining). Credit: Wikipedia

So, what’s it going to take for magnetotactic bacteria to become our microbots of choice? One challenge scientists are now focusing on is 3-D aggregation. Using a single electromagnet, it’s simple business to line a bunch of magnetic bacteria up in a 2-D sheet. Forming a 3-D swarm is more challenging. But if doable, 3-D configurations would have major advantages. A 3-D swarm would be much easier to guide through the intricacies of the human vascular system. It would also be able to build 3-D objects more efficiently.

In a paper recently published in the Journal of International Robotics Research, scientists used Magnetococcus marinus, a spherical, magnetotactic bacterium possessing 2 bundles of flagellar “propellors”, to explore the possibility of creating 3-D bacterial clusters with magnets.

Using remotely powered pairs of electromagnetic coils, the researchers applied three different time-varying magnetic field sequences to a liquid culture of M. marinus. They were able to generate both 2-D and 3-D bacterial configurations. Then they managed to steer these swarms through a complex network of glass tubes intended to mimic capillaries.

These basic steps are laying the foundation of being able to one-day guide armies of magnetotactic bacteria to carry out many tasks. In the future, swarms of magnetic “biobots” might be used for targeted cancer treatments, transporting microscale objects, assembling microcircuits, and even microscale magnetic resonance imaging.

ResearchBlogging.org

de Lanauze, D., Felfoul, O., Turcot, J., Mohammadi, M., & Martel, S. (2013). Three-dimensional remote aggregation and steering of magnetotactic bacteria microrobots for drug delivery applications The International Journal of Robotics Research, 33 (3), 359-374 DOI: 10.1177/0278364913500543

Geobacter, the microbe electric, uses biotic nanowires to breathe iron

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Geobacter metallireducens, an iron breathing bacteria

Geobacter metallireducens, an iron breathing bacteria

While it is hard for most of us to imagine life without oxygen, bacteria have been finding other ways to breathe for billions of years. A particularly successful group of anaerobic (non-oxygen breathing) bacteria is the Geobacter. From acid mine drainage sites to iron-rich rocks buried meters beneath the Earth and rusting ship wreckage on the ocean floor, Geobacter specialize in environments inhospitable for most life. How do they do so? For one, Geobacter don’t need oxygen. They can “breathe” using a number of other elements, including iron, sulfur and uranium. Recently, another startling feature of Geobacter was discovered that may shed light on their success as iron-breathers. They are electrically conductive.

Before we get to electrical conductivity, a bit of background on iron breathing, or “iron reducing”  in technical lingo. All life on earth requires energy. On a molecular level, all life acquires energy in much the same way: stripping electrons from one substance (usually, but not always, carbon) and transferring said electrons to another substance- an electron acceptor. Oxygen is the preferred electron acceptor among multicellular organisms because of its high electron affinity. This just means you get more “bang for your buck” using oxygen to strip electrons off your food than using, say, iron or sulfur. But oxygen is not found everywhere, and many microbes have become adapted to using other electron acceptors in lieu of oxygen. In theory, this makes sense. In practice, iron is a bit of a head-scratcher. In its oxidized (i.e., electron-depleted) form, iron is a heavy, insoluble metal that cannot easily cross cell membranes. For decades, scientists have assumed that iron-reducers like Geobacter have some adaptation that allows them to use iron outside of their cells for respiration.

Long appendates (pili) on Geobacter allow them to conduct electrons to heavy metals in their environment

Long appendates (pili) on Geobacter allow them to conduct electrons to heavy metals in their environment

This is where electricity comes in. Like many bacteria, Geobacter has long, filamentous appendages called pili extending from its body. Pili allow bacteria to sense their environment, similarly to whiskers or antennae. Sometimes, bacteria use pili to directly interact with their environment, releasing chemical compounds or exchanging genetic information with other bacteria. It turns out Geobacter’s pili are highly electrically conductive- as conductive as synthetic organic metals. This discovery has led scientists to hypothesize Geobacter’s pili serve as “electrical wires” that conduct electrons from inside the cell to iron in the environment.

Genetic studies have provided substantial evidence to support the “nanowire” hypothesis. The pilA gene encoding pili proteins is more highly expressed when Geobacter is grown with insoluble iron than soluble iron. That is, there is a direct relationship between pili production and the presence of iron that cannot cross cell membranes. To obtain direct evidence that pili are involved in iron respiration, scientists have created “knockout” strains of Geobacter that lack the pilA gene. Sure enough, pilA knockouts cannot respire insoluble iron, but they still grow using soluble iron that can diffuse across their cell membranes. Experimental evidence from culture studies also supports the link between electrical conductivity and pili. When grown in environments where electrical conductivity may provide an advantage, such as on graphite electrodes, Geobacter produce more pili.

A microbe with metallic conductivity is more than just a curious oddity of nature. Geobacter’s conductive pili raise exciting prospects for engineers in the emerging field of bioelectronics, who envision creating nano-powergrids out of “microbial wires”. Geobacter grown on electrodes may one day serve as a cheap energy source- if we can find a way to harness that energy.