Researchers from the Weizmann Institute of Science have discovered that the movement of bacteria from different species produces spectacular geometric patterns – a result of collective movement that occurs in nature, and stems from unique microscopic swimming properties.
A desert region in northern Mexico ignited the imagination of a professor Joel Stevens, a physicist who studies the physics of biological processes and is interested in studying natural microbial communities. He has studied a unique habitat of bodies of water with conditions similar to those that existed on Earth about 700 million years ago. This area, the Cuatro Sainges (literally: Four Swamps) Basin in the state of Coahuila, has an extraordinary diversity of creatures, including rare microbial communities that thrive in extreme conditions reminiscent of the Earth of the Precambrian era.
Prof. Stevens and Dr. Rinat Arbel-Goren – a faculty scientist from his research group in the Department of Physics of Complex Systems at the Weizmann Institute of Science – joined research expeditions that were organized to collect samples from this area. Later, when they were restored with some of the ancient species in Israel, Dr. Oscar Gayardo-Navarro, a microbiologist familiar with the unique environment, also joined from Mexico.
In one of the experiments he conducted as part of his postdoctoral research at the institute, Dr. Gayardo-Navarro was surprised to discover that, even after repeated attempts, he was unable to evenly distribute the bacterial culture growing in the growth medium. Every time he mixed the culture and observed it under a microscope, cloudy areas and bright areas with patterns typical of each bacterial species quickly formed in the bacterial mixture. The formation of the patterns affected not only the samples, but the entire study: in the articleRecently published In the scientific journal Nature Communications, the research group revealed that different species of bacteria that swim towards oxygen produce unique patterns of self-organization. That is, each of the bacterial species studied created its own unique spatial pattern – some created hexagonal patterns, while others created elongated or twisted structures. These findings shed new light on the understanding of collective movement that occurs in various natural systems – such as flocks of birds, fish and ants, and are also relevant to humans and robots.
"The spectacular patterns observed are the result of a process called bioconvection," explains Prof. Stevens. "Whether in a lake or in the laboratory, the bacteria swim upwards, towards the surface of the water, because there is a higher concentration of oxygen in the air, and they need it to survive. However, because 'bacteria are heavier than water' - meaning their density is higher than that of water - when they reach the surface they sink back down due to gravity and create convection currents. In fact, there are two opposing forces at work here - gravity on the one hand, and the bacteria's biological need for oxygen on the other. In addition, like in a vicious circle, their life up and down perpetuates and strengthens itself through the water currents that are created."
Bioconvection, in itself, is a well-known phenomenon in the scientific literature, but this study revealed a new facet of it: an impressive wealth of patterns that characterize different species. "In the past, researchers focused mainly on models of cultured laboratory bacteria, and no one thought to examine in depth the unique patterns that characterize the collective behavior of bacteria of different species and strains collected from a single community and living together in nature," says Prof. Stevens.
The principle of separation
"Since the different species came from the same habitat and community, we said to ourselves - why not mix them in the lab and see what happens?", says Dr. Arbel-Goren. "At this point, a big surprise awaited us - we discovered that they do not mix with each other, and maintain spatial separation in the culture." The scientists carefully mixed different species of bacteria in varying ratios, and watched in amazement as new patterns of bioconvection emerged as a result of the combination of the unique patterns of each species individually. They marked the different species with fluorescent markers in different colors and tracked with great precision the position of each species within the spectacular kaleidoscopic pattern that was created.
But how and why does this spatial separation between the species occur? The researchers concluded that it does not stem from biological interactions of attraction or repulsion between the bacteria; after all, as long as bioconvection did not occur, there was no sign of spatial separation. Nor could they explain it by the morphological differences between the different bacteria they studied. They hypothesized that this phenomenon might stem from the unique characteristics of the movement of the different bacteria. Therefore, they turned to characterizing the swimming patterns of the different species, and discovered that each species has an average speed and frequency of changes in direction that are unique to it. The more pronounced the differences in swimming properties between two bacterial species were, the greater the spatial separation that was ultimately created. "It is amazing to see how differences in movement on a microscopic scale of about 2 microns are reflected in patterns on a scale of more than 1,000 times larger," marvels Prof. Stevens.
The researchers speculate that the separation arose during evolution to minimize collisions between bacteria swimming alongside each other or to allow each species to best meet its needs in the shared environment. The new findings have implications not only for those species living in the microbial paradise in Mexico, but for bacterial communities all over the world. Moreover, Prof. Stevens explains that the findings may be relevant to the field of what physicists call “active matter,” that is, collective motion resulting from the spontaneous motion of each individual in a system, and the formation of group behavior from the activity of individual actors who do not necessarily communicate with each other.
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