This illustration shows the
vitrification – the transformation into a glasslike state – of E.coli over time as their numbers become more dense
and compacted. Credit: 2024 Tomo Narashima (LAIMAN)
Dense
E. coli bacteria have several similar qualities to colloidal glass, according
to new research at the University of Tokyo. Colloids are substances made up of
small particles suspended within a fluid, like ink for example. When these
particles become higher in density and more packed together, they form a
"glassy state."
When researchers multiplied E. coli bacteria within a confined area, they found that they
exhibited similar characteristics. More surprisingly, they also showed some
other unique properties not typically found in glass-state
materials.
This study, which is published in PNAS Nexus, contributes to the understanding
of glassy "active matter," a relatively new field of materials
research which crosses physics and life science.
In the long term, the researchers hope
that these results will contribute to developing materials with new functional
capabilities, as well as aiding our understanding of biofilms (where
microorganisms stick together to form layers on surfaces) and natural bacterial
colonies.
What do butter, soap and ink all have in
common? They certainly don't all taste good, but they are all types of
colloids, substances made of particles suspended in fluid. When the
concentration of particles is low, then the substance will be more liquid, and
when it is high, then it becomes more solid (think of a dried-out inkwell).
When this happens, the substance enters a glassy state, whereby the movement of the particles is restricted. However, although it may feel hard, unlike with other solids, the particles do not form fixed patterns but are jumbled together randomly. This is similar to the molecular structure of glass.
This video shows the E.coli transitioning into a
glassy state as their density increases and they can no longer move. Credit: 2024 H. Lama, M. J. Yamamoto, Y. Furuta et
al./ PNAS Nexus
Researchers have now found that the
bacteria E. coli can behave in a similar way.
"Since bacteria are very
different from what we know of as glass, it was surprising that many of the
statistical properties of glassy materials were the same for bacteria,"
said Associate Professor Kazumasa Takeuchi from the Department of Physics at
the Graduate School of Science.
"However, the bigger surprise
for us was that in-depth analysis revealed not only a similarity to the
standard properties of glass, but also other properties beyond that. Our
results call for an extension of our current understanding of the physics of
glass."
Takeuchi was inspired to carry out
the experiment after observing the behavior of bacteria in a different study
over 10 years ago. At that time, he saw that when a population of bacteria
became very dense, it abruptly stopped moving and he wanted to understand why.
The main challenge was to create an
environment in which the bacteria could equally thrive and multiply to form a
dense population. To achieve this, the team used a device they had previously
developed, which enabled them to equally distribute nutrients through a porous
membrane to all the bacteria. The researchers then observed the bacteria by
microscope over 5-6 hours.
As the number of E. coli increased, they became caged in by their neighbors, restricting their ability to swim freely. Over time, they transitioned to a glassy state. This transition is similar to glass formation, as the researchers noted a rapid slowdown of movement, the caged-in effect and dynamic heterogeneity (whereby molecules travel longer distances in some areas but hardly move in others).
This color overlay highlights the
orientation of groups of bacteria in different areas. Credit: 2024 H.
Lama, M. J. Yamamoto, Y. Furuta et al./ PNAS Nexus
What
made this bacterial glass different to other glasslike substances was the
spontaneous formation of "microdomains" and the collective motion of
the bacteria within these areas. These occurred where groups of the rod-shaped
E. coli became aligned the same way.
The researchers were also surprised that
the way the bacteria vitrify (turn into a glasslike state) apparently violates
a physical law of typical thermal systems. What we characteristically know as
glass, including colloidal glass, is classed as thermal glass. However,
recently researchers have started to explore glassy states, like the one
reported in this paper, which aren't considered thermal glass but share many of
the same properties.
"Collections of 'self-propelled
particles' like we see here have recently been regarded as a new kind of
material called active matter, which is currently a hot topic and shows great
potential," explained Takeuchi.
"Our results on bacterial glass are
along this line of research, extending this concept to the realm of glassy
materials. In the long term, our results might contribute to developing novel
materials with some functions that are impossible with ordinary
materials."
Next, the team wants to explore how this
phenomenon plays out with other diverse species of bacteria in different
environments. Ongoing research has so far shown that there are different ways
in which cells can become crowded together.
Takeuchi said, "Our results
indicate that dense bacteria can drastically change their mobility and
mechanical properties at the population level, by a minute change in the cell
density. This information could be used to regulate or control dense bacteria
formations in the future. Through our work, we hope to make deeper and broader
connections between statistical physics and life science."
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