World’s First Discovery: Statistical Distribution Rule of "Like Attracts Like" Broken by the Power of Bacteria Smaller Than Dust

UNIST and Stanford Researchers Discover New Distribution Law for Particles in Non-Equilibrium States
Key Factors Determining Particle Distribution Revealed Using Bacillus subtilis Model
Published in Phys. Rev. Lett.

The principle that "like attracts like" also applies in science.

Water and oil do not mix, and the tiny particles floating within them tend to gather in the environment where they are more comfortable, that is, where their energy is lower. This is a rule of distribution.

However, it has been found that even bacteria, which are smaller than dust particles, can break this statistical distribution if they are capable of moving on their own.

On October 12, Professor Jung Junwoo of the Department of Physics at UNIST, Professor Robert Mitchell of the Department of Life Sciences at UNIST, and Professor Sho Takatori of Stanford University announced that their research team has, for the first time in the world, identified the statistical distribution principles followed by self-propelling particles as small as bacteria.

Research team (from left) Ji-Yong Chun, Ph.D. at UNIST (first author), Kyuhwan Choi, postdoctoral researcher at Stanford University (first author), Kevin Modica, researcher at UC Santa Barbara, Junwoo Jung, professor at UNIST. Provided by UNIST

The results of this study were published online in Physical Review Letters, a leading physics journal, on September 16.

According to the research, the factors determining the distribution of living bacteria are their motility and their preference for a particular liquid phase. The force that attracts bacteria to a specific liquid phase acts to confine them there, while the bacteria's motility enables them to escape, creating a competitive relationship between the two factors.

The research team developed a theoretical model explaining the distribution of bacteria by quantitatively analyzing these two forces. Using optical tweezer technology, they measured the force attracting bacteria to a particular liquid phase and found it to be at the level of 1 piconewton (1 pN). A piconewton is ten million times weaker than the gravitational force felt by a single strand of hair. The propulsive force of bacteria is about 10 piconewtons. This means bacteria can overcome the confining force with their own motility.

This study was conducted using an experimental model in which Bacillus subtilis, a bacterium used in Cheonggukjang fermentation, was injected into an aqueous two-phase system composed of dextran and polyethylene glycol solutions.

When dextran and polyethylene glycol are dissolved in water, they do not mix, resulting in two separated liquid phases. The surface of Bacillus subtilis is coated with sugar components, so it is naturally attracted to the dextran solution. However, after surface treatment, its preference can be altered so that it is attracted to the polyethylene glycol phase instead.

In the experiment, non-motile Bacillus subtilis gathered in their preferred phase, while living, motile Bacillus subtilis were evenly distributed in both phases. The even distribution of Bacillus subtilis cannot be explained by "thermal fluctuations" alone, which traditionally govern the distribution of particles.

Ji-Yong Chun, Ph.D. in Physics at UNIST (currently a postdoctoral researcher at Georgia Tech), and Kyuhwan Choi, postdoctoral researcher at Stanford University, participated as first authors in this study.

The research team stated, "Through collaborative research spanning physics, chemical engineering, and microbiology, we have, for the first time, quantitatively analyzed the forces governing the distribution of colloidal particles in non-equilibrium states. This model system will enhance our understanding of non-equilibrium statistical mechanics and the interactions between interfaces and colloidal particles in non-equilibrium conditions." Meanwhile, a non-equilibrium state is one in which energy is continuously supplied or consumed from the outside; in this model, bacteria generate propulsion by consuming energy, thus creating a non-equilibrium state.

Professor Jung Junwoo said, "This study not only provides clues to explain the principles by which bacteria settle in specific tissues within the body, but may also aid in protein purification, biochip design, and the development of micro-robot control."

Schematic diagram of bacterial distribution according to experimental model and motility presence.

This research was supported by the National Research Foundation of Korea, the U.S. National Institutes of Health, the U.S. National Science Foundation, the U.S. Air Force Research Laboratory, and the U.S. Packard Fellowship.