The Geometry of Life: Physicists Determine What Controls Biofilm GrowthThe Geometry of Life: Physicists Determine What Controls Biofilm Growth Biofilms are colonies of bacteria that can be found on implanted medical devices, our skin, contact lenses, and in our gut and lungs. They can also be found in sewers and drainage systems, on the surface of plants, and even in the ocean. A new study published in *Physics* uses physics to investigate how these biofilms grow. The study shows that a biofilm’s fitness—its ability to grow, expand, and absorb nutrients from the medium or substrate—is largely influenced by the contact angle that the biofilm’s edge makes with the substrate. The study also found that this geometry has a greater impact on fitness than anything else, including the rate at which the cells can reproduce. This finding could lead to crucial insights into how to combat biofilms, with applications for human health, such as slowing the spread of infections or creating cleaner surfaces. The research team used interferometry, a widely used imaging technique in physics and materials science, to image biofilms. This technique allowed them to obtain nanometer-scale resolution of bacterial colonies. The team found that the shape of a biofilm changes as it grows. Initially, a biofilm has a spherical cap shape. However, as it grows, it deviates from this shape and takes on a shape that resembles an egg in a frying pan. The team was able to develop a mathematical model to describe the growth of biofilms. The model showed that the key factor in biofilm growth is the contact angle: the angle that the edge of the biofilm makes when it touches the surface it is growing on. This finding is important because it provides a new way to understand how biofilms grow. It could also lead to new ways to control biofilm growth, which could have applications for human health and other areas.
Microscopic image of biofilm on rock. Credit: NASA
From dental plaque to pond scum, biofilms can be found almost everywhere. These colonies of bacteria grow on implanted medical devices, our skin, contact lenses, and in our gut and lungs. They can be found in sewers and drainage systems, on the surface of plants, and even in the ocean.
“Some studies suggest that 80 percent of infections in the human body can be attributed to bacteria growing in biofilms,” says Aawaz Pokhrel, a doctoral candidate at the Georgia Institute of Technology and lead author of a new study that uses physics to investigate how these biofilms grow.
The article “The Biophysical Basis of Bacterial Colony Growth” was published in Physics this week, and it shows that a biofilm’s fitness—its ability to grow, expand, and absorb nutrients from the medium or substrate—is largely influenced by the contact angle that the biofilm’s edge makes with the substrate. The study also found that this geometry has a greater impact on fitness than anything else, including the rate at which the cells can reproduce.
“That was the big surprise for us,” said corresponding author Peter Yunker, an associate professor in Georgia Tech’s School of Physics. “We expected geometry to play a big role, and we thought that figuring out the exact geometry would be important to understanding why the range expansion rate, for example (the rate at which the biofilm spreads across the surface over time), is constant. But we didn’t go into the project thinking that geometry would be the be-all and end-all.”
Understanding how biofilms grow and what factors contribute to their growth rate could lead to crucial insights into how to combat them, with applications for human health, such as slowing the spread of infections or creating cleaner surfaces.
“What excited me was this opportunity to use physics to learn about complex biological systems,” adds Pokhrel, who is also a Ph.D. student in Yunker’s lab. “Especially on a project with so many applications. The combination of the importance to human health and exciting research was really intriguing to me.”
A new method
Although biofilms are ubiquitous in nature, they have proven difficult to study. Because these “cities of microorganisms” are made up of tiny individuals, scientists have struggled to successfully image them.
That changed in 2015, when Yunker wondered whether interferometry, a widely used imaging technique in physics and materials science, could also be applied to biofilms.
“Given my background in physics, I was familiar with its use in materials applications,” Yunker recalls. “I thought it would be interesting to apply this technique more broadly, because we know from decades of physics that surface interfaces contain a lot of information about the processes that create them.”
The technique proved to be simple, effective and time-efficient, and provided nanometer-scale resolution of bacterial colonies. “This essentially allows us to get a super-resolution image of the topography – the shape of the surface of the bacterial population,” Yunker added.
Using interferometry, the team began conducting new biofilm experiments, looking at how the shape of colonies changed over time. Co-first author Gabi Steinbach, a former postdoctoral researcher in Yunker’s lab and now a scientific research coordinator at the University of Maryland, noticed that each colony had a specific shape when it was small: a spherical cap, like a slice of the top of a sphere, or a drop of water. It’s a shape that comes up often in physics, and that piqued the team’s interest.
“A spherical cap is really interesting in physics because it’s an area-minimizing shape,” Pokhrel adds. “I was curious about why a biological material would grow in this shape, and we started wondering if there was some physics behind it — maybe there was some geometry involved. And that led us to the idea that maybe we could develop a model. And that got me really excited.”
A mathematical mystery
However, the researchers quickly hit a snag. “Although we could see that the colonies were initially spherical caps, they would deviate from that shape as they grew,” Pokhrel says. “And the shape they grew in was hard to describe with the existing spherical cap geometry.”
“The center wasn’t growing as fast as it should have to maintain the bulbous cap shape, and we wanted to tie all of that into range expansion (the rate at which the colony spreads across a surface),” Yunker adds. “But we knew that geometry was playing a really important role somehow.”
Finally, Thomas Day, a former graduate student in Yunker’s lab, now a postdoctoral researcher at the University of Southern California and one of the paper’s authors, proposed a curious geometry problem: the napkin ring problem.
“Once we started thinking about the napkin ring problem, we could start developing a mathematical toolkit,” Yunker says, though the solution wasn’t effortless. “We couldn’t find anyone who had ever looked at a napkin ring with a ball cap before, because the application is very rare.”
Pokhrel, along with two co-authors, was responsible for working out the geometry. He found that the cells grew exponentially at the edge of the shape, expanding further into the medium, while the cells in the middle grew upward, creating a shape that resembled an egg in a frying pan – as the white expanded outward, while the yolk only grew larger.
This was a groundbreaking discovery: because the cells in the center only contributed to the height of the biofilm, the team only had to consider the number of cells at the edge of the biofilm and the shape they needed to have in order to grow and spread.
After mathematically modeling their findings, the team found that the key factor was the contact angle: the angle that the edge of the biofilm made when it touched the surface it was growing on. That single geometric quality is even more important to the growth of a biofilm than the speed at which it can reproduce cells.
The connection between physics and biology
All in all, the project took over three years, from concept to publication. “Aawaz really put in an incredible amount of effort to bring this work to fruition,” says Yunker. “It was many years and many, many experiments. But the final product is 100% worth it.”
The team hopes the research will pave the way for future research, which could lead to applications such as controlling biofilm growth to help prevent infections.
“Looking ahead, there are still many research opportunities,” Pokhrel says. “For example, looking at competitive experiments between biofilms: do taller colonies change their contact angle so they can spread faster? What role does this geometry play in competition?”
“Biology is complex,” Yunker says. In nature, the surface on which a biofilm grows may not be as consistent as a laboratory surface, and colonies can have different mutations or be made up of more than one species. “But we first had to understand what happens when the temperature and nutrient availability are stable.”
Although the model is based on how biofilms behave in a controlled laboratory environment, it is an important first step in understanding how they behave in nature.
More information:
Aawaz R. Pokhrel et al, The biophysical basis of bacterial colony growth, Physics (2024). DOI file: 10.1038/s41567-024-02572-3
Offered by Georgia Institute of Technology
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