Swimming Cells

As if fluid mechanics wasn’t complicated enough, at the Guasto Lab, it’s scaled down to the size of microorganisms. Associate Professor Jeffrey Guasto and his team explore at a variety of different concepts at this level, one of them being how cells “swim.”

To get from one point to another in a liquid, different cells use a variety of techniques. The focus of Guasto’s recent publication focused on cells with two thin arm-like structures, or biflagellate cells, using a breaststroke-like motion to propel themselves forward. While this is well understood in spaces with uniform liquids, research is just now emerging when it comes to liquids that change in properties.

Picture of C. reinhardtii, the biflagellate used in the experiment

In the real world, cells rarely move through uniform fluids, and often they vary in viscosity (the internal friction or “stickiness” of a fluid). In a 2020 publication titled “Viscophobic turning dictates microalgae transport viscosity gradients,” Guasto and his lab discussed the mechanics of biflagellate cells swimming across a viscosity gradient (a fluid that slowly changes in stickiness).   

When first approaching this topic, a major challenge was that the only quantitative efforts to model cell movement through viscosity gradients dated back to the 70s and 80s. “I think that the folks that did this work are very admirable, but they were limited by the technology that they had at that time,” Guasto explained. As a result, scientists then had conflicting conclusions of how cells were getting around in a viscosity gradient- some were swimming toward areas of high viscosity (the “stickier” areas) and others away from it.

Measured average paths of cell over time (colored lines) in viscosity gradient (shown by shaded background color)

When Guasto’s team approached it analytically, they predicted that cells would collect in areas of higher viscosity, because the cells there would move slower. The way Guasto explained it was that “if you go for a walk out in the snow, you walk slower than if you were walking on pavement.” However, after the initial experimentation, the lab observed more cells in areas of lower viscosity, contradicting the original hypothesis. “What's interesting about this is that we don't get what we expect. That means that there's new physics we didn't think of, so that gets very exciting.”

With this new discovery, the team set to work observing cells to get a better understanding as to why this is happening. “Some of the questions that we ask ourselves are: is this physics, or is this a biological response?” Guasto notes. To figure this out, they looked at exactly how the cells were swimming, and what directions they were heading in.

In random motion, cells travel in all directions and are constantly changing. In an area with consistent viscosity, the cells spread out uniformly. However, if biflagellate cells are swimming parallel to the viscosity gradient, each flagellum could be in a fluid of a different thickness.

Guasto realized that this was the key to figuring out why the observations were not what they anticipated. “Imagine you're in a rowboat right, and one of your oars is in water, and one of the oars is in a very thick fluid. The oar that's in the thicker fluid you get a lot more traction on.” There is greater propulsion on the oar that is in the thicker fluid, resulting in a torque that would turn the boat. The same thing is happening in these swimming microorganisms; the increased traction on one of the flagella is causing the cell to rotate away from the higher viscosity areas. The researchers named this phenomenon “viscophobic turning,” and it was recently published in Nature as fluid mechanical explanation for the accumulation of cells in areas of lower viscosity.

Still of a measured fluid velocity field around a biflagellate swimming breaststroke

Although until this point Guasto has mainly dealt in the theoretical related to this topic, there are applications in fields ranging from medicine to environmental protection. “Not all cells are swimming around in some pristine swimming pool,” Guasto explains. Mucus covering the organs in the body, or that found covering corals in the ocean creates a viscosity gradient that cells need to get through to transport nutrients. If we understand how cells navigate a viscosity gradient, we could potentially change the properties of mucus to inhibit or enhance reproduction, reduce the risk of bacterial sicknesses, or the change the amount and type of nutrients that get to corals. There are even potential applications for tiny robots using cell motility to deliver drugs to exactly where we want them to go.

Moving forward, the Guasto lab plans to explore more types of cells and viscosity gradients, and how their discoveries affect our understanding of the biological processes that occur in nature and the human body. Even with the excitement of the wide variety of applications, the study is also an incredible way to see how science and our understanding of it is constantly evolving.