Stressed-out E. coli recovers its straight, rod-like shape over time. [Lars D. Renner/Leibniz Institute of Polymer Research and Max Bergmann Center of Biomaterials, Dresden, Germany]
Architecture isn’t limited to popular buildings from noted designers such Frank Gehry and the late Zaha Hadid, or great structures from ancient civilizations that remain standing. Form and function are rooted in the biological world, which often leaves scientists questioning how a particular cell or organelle came upon its current morphology. This is the question that investigators at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) recently asked about various bacteria, uncovering that Escherichia coli may use mechanical cues to maintain their shape. Findings from the new study were published recently in Nature Microbiology in an article entitled “Mechanical Strain Sensing Implicated in Cell Shape Recovery in Escherichia coli.”
“This research may reveal some basic principles of bacterial growth,” stated co-lead study investigator Felix Wong, a graduate student at SEAS.
Bacteria have an extraordinary ability to maintain and recover their morphology, even after being twisted out of shape. Researchers have understood for some time that shape is determined by the cell wall, yet little is known about how bacteria monitor and control it. Since the cell wall is the target of most antibiotics, understanding how bacteria grow their cell walls may provide insight into more effective medicines.
“We showed that the coupling of cell wall growth to mechanical strain is quantitatively consistent with how bacteria recovered their shape after being deformed in experiments,” Mr. Wong added.
The research team began their efforts by modeling the mechanics of the E. coli cell wall under constraints that forced the bacteria to grow into the shape of a donut. Previous research by members of the SEAS team showed that under similar bending forces bacteria become plastically deformed, meaning when the bending force was removed, E. coli cells snapped back to a straighter, but still bent, shape. This suggested that cell wall growth could sense the applied bending force. However, an observation that was left unresolved in the previous study was that the cells straightened upon further growth.
A theoretical model (left) quantitatively predicts how E. coli (right) grown in confined, microchambers recover their straight, rod-like morphologies over time. [Video courtesy of Harvard SEAS].
In the current study, the researchers explored whether coupling wall growth to mechanical strain—how a bacterium is compressed or stretched—could explain the snapback and predict how fast the bacteria would straighten when released.
We probed “the effects of mechanical strain on cell shape by modelling the mechanical strains caused by bending and differential growth of the cell wall,” the authors wrote. “We show that the spatial coupling of growth to regions of high mechanical strain can explain the plastic response of cells to bending and quantitatively predict the rate at which bent cells straighten. By growing filamentous Escherichia coli cells in doughnut-shaped microchambers, we find that the cells recovered their straight, native rod-shaped morphologies when released from captivity at a rate consistent with the theoretical prediction.”
First, the research team used a theoretical model to quantitatively predict how the bacteria would grow to recover their straight shape and how long it would take. The investigators subsequently ran the experiment with E. coli and found that their models and experiments were consistent with each other. A mechanical strain-dependent cell wall growth rate predicted a straightening rate consistent with what was found experimentally.
“We think our proposed idea for bacteria is reminiscent of plant growth,” concluded Mr. Wong. “It's been well established in the plant field that mechanical cues can bias plant growth. Our research shows that the same may well be true for bacteria. However, if mechanical strains were indeed an important sensory cue for bacteria, then there has to be a molecular mechanism that senses mechanical strain.”