In 1970, Richard Blakemore noticed bacteria he had taken from Woods Hole in Cape Cod, Massachusetts, behaving strangely. The bacteria followed the pull of a magnet. They were the first of their kind found in the United States. Blakemore, then a microbiology graduate student, called them “magnetotactic” for their movement, or “taxis,” along Earth’s magnetic field lines.
A few decades later, Alexander Petroff was in graduate school at the Massachusetts Institute of Technology (MIT), developing an interest in experimenting in microbiology through his lens of physics. He learned to cultivate different bacteria at a summer camp on the site where Blakemore had found magnetotactic bacteria.
Now a professor at Clark University in Worcester, MA, Petroff studies Multicellular Magnetotactic Bacteria (MMBs), a species of magnetotactic bacteria found at Woods Hole. For the past few months, I, along with a few other students, have had the privilege of working in Petroff’s laboratory.
“There weren’t that many papers when I got started,” Petroff says. Microbiology is “a place that a lot of physicists, until recently, have been sort of careful about entering because it’s very messy.”
To keep the MMBs’ movement authentic, Petroff sources the bacteria directly from their habitat, which complicates experiments. Naturally sourced bacteria differ in size and speed, so we use population averages. The amount of bacteria available in our laboratory is also inconsistent. If we inject too many, we struggle to track the masses. After about three months, we ran out of bacteria and returned to Woods Hole to collect more.
On a Cape Cod summer day, we step into waders and slosh through the salt marsh. Petroff points out pink flecks which line the pond floor, indicating the presence of bacteria. We scoop bucketfuls of surface-level mud and seawater. Our bacteria occupy the top few centimeters of sediment, which contain the necessary living conditions for them.
MMBs contain magnetosomes, which are crystals made of iron found in their environment. Magnetosomes give each bacterium a magnetic moment, defining how quickly the bacterium aligns with Earth’s magnetic field lines. The magnetosomes work independently of the bacteria, so even after a bacterium dies, it remains oriented north. Earth’s magnetic field lines run between Earth’s North and South Poles, so in Cape Cod in the northern hemisphere, the field lines are at an angle off the surface of the Earth, allowing our north-seeking bacteria to swim downwards through the sediment.
MMBs are a rare example of evolutionary optimization. It is difficult to find living things with perfect characteristics. “My hands aren’t absolutely uniquely optimized for turning tiny screws,” Petroff explains. They are “just good enough” for the many things we use our hands for. Luckily, our bacteria are optimizing for one goal: to swim through their maze of sand as fast as possible. To study this, Petroff designed a maze made from a silicone polymer, with pores the scale of micrometers—the size of the spaces between sand grains. The pores are shaped to trap bacteria just as biofilm growing between sand grains would in nature. This chamber fits neatly on a slide to be inserted into a microscope. Coils of wire, custom-designed to fit the microscope, produce a magnetic field which directs the MMBs through the chamber.
In a very strong magnetic field, the bacteria are pushed against the wall of the pore they are in, unable to jump away and escape. When the force of the field is weak, the bacteria scatter around the pore randomly. Either way, they have a hard time finding an escape to the next pore. For MMBs to get through the sediment as quickly as possible, their magnetic sense of direction is only one part of the equation.
Petroff predicts “magnetotactic bacteria have evolved to optimally balance magnetotaxis with obstacle avoidance.” Mathematically, a ratio of physical characteristics, including the size of the bacterium, its speed, and its magnetic moment, seems to relate to these environmental factors. For example, a bacterium that swims faster than average must have a larger magnetic moment, so that after bouncing off a wall, it realigns with the magnetic field as fast as it swims.
With an idea of the equation for this relationship in mind, Petroff creates a simulation of MMBs in sediment and fits the data to the function describing the ratio. This model aligns with our experimental data, confirming his hypothesis. Evolution found the perfect combination of physical traits that would get MMBs through their environment as quickly as possible. In fact, other species of magnetotactic bacteria might be optimized as well. For a large variety of sizes, speeds, and magnetic moments, the other variables adjust to this optimal ratio.
“There is deep math that you can do in simple biology,” Petroff has found. “I think there is a place for people who are really interested in physics to use simple arguments… to learn how evolution shapes microorganisms.”
We are at the beginning of studies in physics on magnetotactic bacteria. Experimenting with microbes like MMBs is challenging and requires creative solutions, yet bringing the aspect of physics into microbiology can answer questions about why evolution makes some organisms perfect for a specific goal. “I was sort of charmed by them,” Petroff says, a naturalist at heart. His work is integral to understanding what he calls “profoundly odd creatures.”
