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Scientific Frontline: Extended "At a Glance" Summary: Bacterial Morphogenesis in Liquid Crystals
The Core Concept: Bacteria growing within an aligned liquid crystal fluid—environments mimicking specific biological settings like biofilm matrices or mucus linings—organize into single-cell-wide chains that gradually lengthen before experiencing localized buckling, ultimately forming a tangled, serpentine network.
Key Distinction/Mechanism: Unlike bacteria in random polymeric fluids, which form multi-cell-wide "living gels," those in liquid crystals are forced into single-file alignment by the "bending elasticity" of the surrounding molecules. As the chain grows, viscous drag creates a compressive force, causing the chain to buckle sharply in localized regions rather than bowing along its entire length, as this minimizes the energy cost of disrupting the aligned liquid crystal molecules.
Origin/History: Published in the journal PNAS by Sujit Datta (Caltech) and collaborators from Princeton University, the University of Wisconsin–Madison, and the University of North Carolina at Chapel Hill. The research builds upon prior studies of bacterial growth in unaligned polymeric fluids.
Major Frameworks/Components:
- Bending Elasticity: The energetic tendency of aligned liquid crystal molecules to resist misalignment, which forces the bacteria into single-file chains and localizes their eventual buckling.
- Viscous Drag and Compressive Force: The high viscosity of the liquid crystal fluid creates drag as the bacteria divide and lengthen, resulting in an internal compressive force that drives the buckling.
- Mathematical Modeling: The application of fluid dynamics and elasticity physics to self-replicating biological systems to predict morphological outcomes.
Branch of Science: Biophysics, Microbiology, Applied Mathematics, and Fluid Dynamics.
Future Application: The findings could inform the development of engineered living materials where bacteria are used as sensors or microscopic structural organizers. It also suggests that single-cell chain growth may assist bacteria in navigating microscopic crevices within host tissues or soils.
Why It Matters: Understanding how complex environments dictate the physical structure of bacterial colonies provides crucial insight into their biological functions, including intercellular communication, nutrient acquisition, and antibiotic tolerance.
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| First, bacteria in a liquid crystalline environment grow in a single-file chain. The scale bar shown here is 50 microns. Image Credit: Gonzalez La Corte et al. PNAS/Caltech |
When scientists study bacteria that are not in a petri dish or a test tube but in environments that more closely mimic their actual microbial home settings, they often find delightfully unexpected behaviors. For Sujit Datta, a professor of chemical engineering, bioengineering, and biophysics at Caltech, those oddities are an invitation to apply physics to exciting new puzzles. The results reveal previously unknown ways in which bacteria organize themselves and interact with their surroundings and suggest new questions about how the shape of bacterial colonies affects their ability to survive, grow, and even resist treatment.
Recently, Datta and his former graduate student Sebastian Gonzalez La Corte looked at bacteria growing in liquid crystal fluids—those whose molecules are elongated and all point in the same direction. This aligned state is known to occur in some biological fluids, such as certain biofilm matrices and mucus linings of the airways and gut, yet laboratory experiments typically study bacteria in fluids whose constituents have no preferred direction. Once again, the team made some unexpected observations.
Datta's group previously studied and modeled bacterial growth in polymeric fluids with randomly arranged molecules. In that setting, the team found that several commonly studied bacterial species, including Escherichia coli, Vibrio cholerae (the pathogen that causes cholera), and Pseudomonas aeruginosa (a species that often causes infections in immunocompromised or hospitalized patients), grow into long cables, several cells wide, that intertwine and form a kind of "living gel."
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| Eventually, when the growing chain reaches a certain length, the drag from the viscosity along the chain's length becomes too great, and it buckles in a localized manner. Image Credit: Gonzalez La Corte et al. PNAS/Caltech |
In contrast, the new work shows that in an aligned liquid crystal fluid, bacteria build single-cell-wide chains that grow and lengthen in relatively straight lines until they suddenly buckle in an unexpected way.
When a steel beam is compressed on either end, it eventually buckles, bending gradually in an arclike fashion along its entire length. But when the "beam" is made of bacteria in a liquid crystal fluid, its buckling is localized—with only a small section bending very tightly. Why would these bacterial beams behave so differently?
"This is a weird mechanics problem. Mechanicians have thought about thin, slender beams buckling for decades," Datta says. "But this is an odd version of that problem because this beam is made out of cells that are self-replicating."
Datta and his colleagues collaborated with applied mathematicians at the University of Wisconsin–Madison and the University of North Carolina at Chapel Hill to build a mathematical model of bacterial growth in liquid crystals. To understand the system, Datta says to consider the metaphor of matches in a matchbox; the matches are the aligned liquid crystal molecules. Now imagine the introduction of a bacterium into the system; the bacterial cell, which is much larger than the liquid crystals, could be represented by a pencil in a very large matchbox.
"The matches don't want to be bent out of their aligned orientation, so they push back. They force the pencil to be aligned with them," Datta explains. This is what scientists refer to as "bending elasticity" in the aligned liquid crystal, and it guides much of the bacterial behavior the scientists observed in the system.
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| This 3D confocal reconstruction shows a colony of nonmotile E. coli bacteria after 520 minutes, showing the single-cell-wide chains that have buckled and twisted, forming a serpentine structure. Image Credit: Gonzalez La Corte et al. PNAS/Caltech |
Continuing the metaphor, now imagine the pencil is growing and dividing end on to produce more pencils, which also end up aligned and form a single chain—the most energetically favorable scenario, Datta says. At the end of the growing pencil in the box, the matches must bend slightly to accommodate the width of the pencil, and that bending costs the system energy. The bending energy penalty is steeper if multiple pencils are stacked up to form a thicker chain.
All of this became very clear in the new model, which showed that the liquid crystal molecules push on the bacteria, maintaining the observed single-cell-wide chain. In combination with the well-established physics of liquid crystal behavior, the team was able to quantitatively predict the extent to which a bacterial chain will be aligned in a liquid crystal fluid.
The bending elasticity of liquid crystals also helps explain the unusual buckling the scientists observed. When bacteria grow in liquid crystals, the high viscosity of the liquid crystal fluid causes a buildup of drag, a compressive force along the entire length of the bacterial chain. Unlike a steel beam pressed at its two ends, the chain squeezes itself: Every cell divides, so the chain lengthens, causing the fluid drag to increase until eventually the compressive force is so strong that something has to give. But the liquid crystal molecules also resist being pushed out of alignment. In the case of normal buckling, all the liquid crystal molecules along the bacterial chain would become misaligned and bend a bit along the entire length of the chain. When the buckling is more localized, the liquid crystal molecules bend significantly, but only in a small region. It turns out this latter scenario is energetically less costly than the situation where all the molecules bend.
Using confocal microscopy, the researchers observed that the chain of bacterial cells buckled multiple times, creating these tight little kinks. Eventually, the buckled chains built up a tangled, serpentine network.
"We know that the spatial arrangement of bacteria influences their biological function—how they communicate with each other, how they access nutrients, and how they tolerate antibiotics. That's why being able to understand what sets the morphology of a colony is important," Datta says.
The biological implications of these observed and modeled microbial behaviors are not yet known. But now that they know about this type of growth, researchers can pose and explore a host of additional testable questions, Datta says, such as whether single-cell chain growth might help bacteria snake through tiny crevices and colonize crowded environments—for example, host tissues or environmental soils.
The growth model also suggests a possible route toward the development of engineered living materials that incorporate bacteria for use as sensors or building materials. Growing bacterial cells inside liquid crystal environments could allow biology to do the work of organizing a material at the microscopic scale, Datta says.
More broadly, Datta says, the work provides another piece of the larger puzzle of how bacterial life is shaped by the complex environments it typically inhabits. "This is yet another environmental complexity that bacteria often have to contend with, and we just didn't know how it would alter their growth as colonies," he says. "Now we know not only how it affects that, but we can make sense of it in quantitative and predictable ways."
Funding: The work was supported by the National Science Foundation (NSF), the Camille Dreyfus Teacher-Scholar and Pew Biomedical Scholars programs, the National Institutes of Health's National Institute of General Medical Sciences, and the Center for the Physics of Biological Function at Princeton University. It was initiated in part at the Aspen Center for Physics, which is supported by the NSF.
Published in journal: Proceedings of the National Academy of Sciences
Title: Morphogenesis of bacterial colonies in liquid crystalline environments
Authors: Sebastian Gonzalez La Corte, Thomas G. J. Chandler, Saverio E. Spagnolie, and Ned S. Wingreen, Sujit S. Datta
Source/Credit: California Institute of Technology | Kimm Fesenmaier
Edited by: Scientific Frontline
Reference Number: biph083126_01
