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The Rigakubu News
Published in The Rigakubu News November 2024

The Frontiers of Research for Undergraduates >

When Bacteria Dense Together, They Form Glass

Kazumasa Takeuchi, Associate Professor, Department of Physics

Matter is a collection of countless atoms and molecules. Many of the phenomena of life are carried out by countless collections of cells.
Can a collection of cells change its state into gas, liquid, or glass like matter?
This may sound like a crazy question, but it is one of the most advanced problems being addressed in the field of "active matter physics," which is at the interface between physics and the life sciences.
We have observed the process of bacterial population densification by growth and found that the state of the population changes from liquid to glass.
What are the common properties of matter and the glass of life? What are the differences? Does life take advantage of the glassy state?
The mysteries are endless.

Living things use whatever is available. Not only the elaborate design and control of genes, but also the physical phenomena that occur when molecules and cells come together are used skillfully. Liquid-liquid phase separation in cells, which has been the subject of active research in recent years, is a prime example. Then, what kind of physical phenomena are produced by the assembly of biomolecules and cells? It makes a certain amount of sense to investigate the contents of the "toolbox of physics," so to speak, that life possesses. Active matter physics studies the collective properties of particles that mimic biomolecules, cells, and individual organisms. By contrast with matter, which is a group of atoms and molecules, active matter physics has shed new light on the view of matter by developing the characteristics of biological particle populations that are different from those of matter. This paper introduces the discovery that biological populations can also become glass.

In everyday life, the term "glass" often refers to materials composed mainly of silicates, but in physics, a glass is a state in which a large number of particles, whether molecular or colloidal, are crowded together and remain stuck in a disordered state. In fact, many materials other than silicates also undergo supercooling and become glassy when rapidly cooled from a liquid state, and in the case of colloids, if the particles are packed densely enough, they become glassy. What about life?

In fact, in life phenomena, congestion is everywhere. Inside a cell, countless bio-molecules are crowded together. Cellular tissues are made up of many cells tightly packed together. Bacteria often live in dense clumps called biofilms. However, it is not easy to compare biofilms with glasses made of ordinary materials, and the characteristics of biofilms have been unknown.

We have fabricated a microfluidic device for high-density culture of bacteria and succeeded in observing the process of crowded growth of Escherichia coli under uniform conditions. The E. coli, which were actively swimming around at low density, become densely populated by proliferation and become stuck like a crowded train. The analysis revealed various properties common to glasses of ordinary materials, such as a rapid increase in structural relaxation time and the appearance of dynamic heterogeneity. On the other hand, properties unique to biological populations also emerge. For example, unlike conventional molecules, E. coli swims by itself. Under crowded conditions, neighboring bacteria try to move in groups to some extent in order to align their body orientations. Perhaps this is the reason why we detected statistical features that are different from those of normal glasses. In addition, the way the relaxation time increased violated the restrictions that normal glass must adhere to. It seems that living organisms can be vitrified in a greater variety than ordinary materials.

(a) Membrane-type microfluidic device used in the experiment. (b) Vitrification of E. coli population. The transition from the liquid state to the oriented glassy state to the fully glassy state occurs over time. The color represents the orientation of the cell body. You can see the video in the press release.

It is not known whether life takes advantage of the glassy state. However, the characteristics of glass, in which slight differences in density can drastically change the flow and mechanical properties, may be of use. Inside a cell, molecules are pressed into the cell at concentrations that would normally cause it to vitrify. It has been reported that bronchial epithelial tissue is vitrified to different degrees in asthmatics and normal subjects. The role of glass in life may be more important than we thought.

The results of this study were published in H. Lama et al, PNAS Nexus, 3, 238 (2024).

(Press release, July 11, 2024)