Researchers at Caltech and Princeton University have made an intriguing discovery regarding bacterial cells that thrive in polymer-rich environments, such as mucus. They observed that these bacteria form extended, intertwining cable-like structures, reminiscent of “living Jell-O.”
This discovery holds significant potential for understanding and treating conditions like cystic fibrosis, where thicker mucus in the lungs often leads to serious bacterial infections. It may also shed light on biofilms—clusters of bacteria encased in gel-like substances—found in natural settings like river rocks and in industrial contexts where they can lead to equipment failures and health risks.
The research findings were published on January 17 in the journal Science Advances.
According to Sujit Datta, a chemical engineering, bioengineering, and biophysics professor at Caltech and lead author of the paper, “When many bacteria grow in watery fluids that contain long-chain molecules, like the polymers found in mucus, they create intertwined structures resembling living gels. Notably, the physics behind this process is similar to that driving the creation of many nonliving gels, such as Jell-O or hand sanitizer.”
Datta, who recently transitioned to Caltech from Princeton, worked with graduate student Sebastian Gonzalez La Corte, who is the lead author of the publication. They were exploring changes in mucus concentration in cystic fibrosis patients, who have higher polymer levels than usual. Utilizing mucus samples from MIT, Gonzalez La Corte grew E. coli in standard liquid and in mucus-like solutions, observing the bacterial growth under a microscope.
Focusing on non-motile bacterial cells, which are unable to swim, Gonzalez La Corte noted that as these cells multiplied, they typically drifted apart. Yet, in a polymeric solution, the replicated cells remained connected end to end.
“As cells keep dividing and attaching to one another, they create stunning long structures we refer to as cables,” Gonzalez La Corte explained. “Eventually, these cables begin to bend and intertwine, forming a complex network.”
The researchers observed that these cables continue to grow and expand, as long as the cells have a sufficient nutrient supply, eventually producing chains that can contain thousands of cells.
Further investigations revealed that the type of bacterial species or the specific polymer solution did not impact the formation of cables; once there was ample polymer surrounding the cells, the cables would develop. They even replicated these findings with synthetic polymers.
While the study initially aimed to improve understanding of infections in cystic fibrosis patients, its implications extend further. Mucus plays a vital role in various parts of the human body, including the lungs, gut, and cervicovaginal tract. Datta also emphasizes the relevance of the findings in the context of biofilms—aggregates of bacteria that secrete their own polymer matrix. Biofilms not only exist in the human body, like dental plaque, but are also prevalent in soil and industrial environments, where they can cause harm to machinery and pose health risks.
“The polymer matrix secreted by biofilms makes them very difficult to remove from surfaces and resistant to antibiotics,” Datta points out. “Understanding how cells grow within this matrix could be crucial for finding more effective ways to manage biofilms.”
Understanding the Physics Behind the Cables
The research team conducted meticulously crafted experiments that revealed the external pressure from the surrounding polymers is what drives the bacterial cells together and maintains their position. This attractive force, influenced by external pressure, is known as a depletion interaction in physics. Gonzalez La Corte developed a theoretical model of bacterial cable growth based on this theory, enabling predictions about when cables are likely to survive and grow in a polymer-rich environment.
“Now, we can apply established theories from polymer physics, originally created for entirely different phenomena, to these biological systems to quantitatively determine when these cables could emerge,” Datta remarked.
Why Do the Bacteria Form These Cables?
“We’ve stumbled upon this intriguing and unforeseen phenomenon,” Datta explained. “We can elucidate its mechanics from a physics standpoint, but the biological implications remain uncertain.”
There are two primary theories regarding the cable formation: bacteria may be clustering to enhance their size, making it more challenging for immune cells to engulf and destroy them. Alternatively, this cabling might be detrimental to the bacteria as these aggregations are prompted by host secretions. “Mucus is dynamic; for instance, in the lungs, it’s regularly moved by tiny hair-like structures towards the throat,” Datta noted. “Is it possible that when bacteria form these cables, it actually facilitates their expulsion from the body?”
At this point, the true rationale behind this phenomenon is still undetermined. Datta says that this uncertainty is what keeps the research engaging. “Now that we have identified this phenomenon, we can pose new questions and devise further experiments to explore our hypotheses,” he concluded.
