Oscillations are ubiquitous in nature — ranging from fireflies flashing in synchrony to brain cells firing in harmony. The latter is essential for several cognitive brain functions such as memory, recall, and attention. And in most cases, such collective behavior requires the disparate units or particles to be coupled or connected, sometimes over long distances through diffusive chemicals or biomechanical interactions. In each of these cases, trying to understand the mechanism behind a collective oscillatory pattern is a challenge.
Figure 1. Collective oscillation in dense bacterial swarms as revealed by silicone oil tracers. Red traces mark the tracer trajectories. Image credit: Yilin Wu
But what of synchrony between units without any “coupling” – a biological word for connectivity? Now, for the first time, an international team of scientists has shown
that within a population of millions of motile bacterial cells, bacteria are able to organize themselves into oscillatory motion without any long-range connections! “Individual bacteria, on the micrometer scale, are characterized by disordered motion, but on the very large scale—near millimeter—the average velocity shows unexpected regular oscillations,” says Francesco Ginelli, reader at the Department of Physics in the University of Aberdeen, an independent expert, not part of the reported study.
Also, “the discovery is a unique 'weak synchronization' mechanism in the sense that it does not require inherent oscillation of individual cells,” says Yilin Wu, assistant professor of physics at The Chinese University of Hong Kong and a senior author of the study published in Nature. Single bacterial cells in the midst of a dense population seem to move in an erratic manner but all together have been described to be taking part in a “flash mob”, moving collectively in an elliptical loop, possibly taking cues from their local neighbors.
This interesting bacterial behavior could be one of the key findings in the field of active matter systems – a hot topic in statistical physics. Active matter has caused scientists to grapple with some key questions, such as, could a chaotic motion in a microscopic scale give rise to long range macroscopic order? “In simpler terms, there is a subtle kind of large-size order that cannot be appreciated by looking at the system too closely,” says Ginelli, adding that “thinking of an impressionist painting could be a useful metaphor!”
Looking into the origins of this study, one discovers a fitting alliance between experimental and theoretical sciences. At a 2015 conference Hugues Chaté, a senior scientist at CEA-Saclay in France, heard Chong Chen (the study’s first author) talk about his observation of a strange synchronous motion of bacterial cells. “I remember thinking, “Am I really understanding what he is saying or is it because I was not listening very well at the beginning?’,” laughs Chaté, to whom this strange bacterial behavior was quite reminiscent of what he had seen in a theoretical lattice model with self-propelling particles. And it is not everyday that a theoretician gets an opportunity to validate his results with experiments, that too from a totally different field! After the talk, Chaté approached Chen to discuss the results in more detail. The rest, as they say, is history! Chaté’s theoretical expertise on simple active matter systems and collective behavior in dynamical systems helped explain the strange bacterial behavior on a global scale, leading to a fruitful collaboration.
Video.Collective oscillation in dense bacterial swarms as revealed by silicone oil tracers.Two silicone oil tracers (dark spots near the center of the image) underwent synchronized oscillatary motion in elliptical trajectories, revealing collective oscillation in the bacterial swarm. Video credit: Yilin Wu.
“Performing interdisciplinary research is particularly important for understanding biological systems, or living matter in general. And perhaps having an interdisciplinary mindset is more essential, as it allows one to explore the field from new perspectives,” remarks Wu.
The conclusions from this study have important applications in fields with self-organizing systems such as swarm robotics. “Think of a swarm of nano-robots injected into your blood-stream for medical purposes. You may wish to use them—for instance—to transport chemical agents the right way and specifically to their intended target, with higher precision and minimal adverse effects,” says Ginelli.
Learning to control biological and artificial active matter systems is one of the critical challenges for the future of this field. “Typically, controlling implies imposing some kind of order to the system, so that it can perform according to your wishes,” adds Ginelli. When bots could potentially be the future of everything from revolutionizing healthcare to grocery delivery, such studies that shed light on how systems work, could be a promising way to inspire new strategies and make a huge difference to our quality of life!
||Lakshmi Chandrasekaran received her Ph.D. in mathematical sciences from the New Jersey Institute of Technology. She is currently pursuing her masters in science journalism at Northwestern University, and is a freelance science writer whose work has appeared in several outlets. She can be reached on Twitter at @science_eye.