Being a member of a community often requires sacrifice. Species from across the animal kingdom have been shown to cooperate with their brethren, contributing resources to serve the greater good even at significant personal cost. But not everyone does their part. Non-cooperative individuals or “social cheaters” can exploit the social contract, reaping the benefits of society while providing little in return.If allowed to run rampant, this selfishness could result in a “tragedy of the commons,” potentially leading to the collapse of an entire population.
Yale biologist Jing Yan, a BWF Career Awards at the Scientific Interface (CASI) recipient, has been studying how bacterial communities avoid this outcome. Bacteria form a special type of collective known as biofilms, assembling themselves into a slimy protective coat to defy the effects of antimicrobial agents, the host body’s immune system, and other threats. If you have ever noticed a green sheen on a vinyl shower curtain or had a dental hygienist scrape layers of plaque off your teeth, you’ve encountered biofilms in real life.
Yan recently teamed up with fellow CASI recipient Jeff Tithof, a mechanical engineer at the University of Minnesota, to investigate how biofilm-forming bacteria keep cheater cells in check. To answer that question, Yan and Tithof drew expertise from a variety of disciplines, including microbiology, molecular biology, physics, and engineering. Their findings, published this summer in the Proceedings of the National Academy of Sciences, could lead to innovative methods to disrupt the biofilms that underpin serious health issues like infections and antibiotic resistance.
From challenge to opportunity
In April 2020, Yan and Tithof, along with the rest of the CASI recipients, received an email from Senior Program Officer Victoria McGovern asking if they were interested in participating in a career development workshop.
“Starting a lab is disconcerting all by itself,” McGovern said. “When COVID-19 disrupted that, we wanted to do something to help new and incoming faculty get a sense of their own balance, and of how important they are to BWF.”
“Everything had just been upended by Covid,” added Tithof. “The idea was to talk about some of the challenges, and work together on how to get through this time.”
The workshop series, called “Full Speed Ahead,” provided much-needed moral support, while also fostering future collaborations. After one Zoom session, participants were asked to connect with other researchers in the program about potential opportunities to work together. Yan quickly fired off a message to Tithof about helping him tackle the problem of biofilms.
“This project has been in my mind for a really long time,” said Yan. “But I’m purely an experimentalist and needed computational tools to see this through. I had been looking for collaborators when I met Jeff and learned that he had expertise in fluid mechanics, I knew I had to reach out to him.”
Yan’s laboratory focuses on how bacteria collaborate to build biofilms. With imaging techniques and mechanical measurements, he has mapped the cellular ordering inside the biofilm clusters of Vibrio cholerae, the aquatic bacteria that cause the diarrheal disease cholera. Tithof’s work deals not with bacteria, but with the brain. He relies heavily on computer simulations, which he uses to understand the glymphatic system, an important waste clearance pathway in the brain relevant to neurological disorders like Alzheimer’s disease.
“We started chatting and realized that we each had specializations that could be very complementary,” said Tithof. One of his postdocs, Saikat Mukherjee, had just begun running simulations of what Tithof called “an idealized but complicated geometry” modeling the flow paths of fluids through the glymphatic system. It was possible that similar simulations could model the flow that Vibrio cholerae biofilms experience in their freshwater habitats or even in their second home, the human gut.
The evolutionary pressure of exploitation
A key feature of biofilms is the 3D extracellular matrix, which binds bacterial cells together and allows them to stick to surfaces without being washed away. To create this extracellular matrix, bacterial cells must collectively produce and secrete glue-like adhesion molecules. That production is costly.
When Jung-Shen Benny Tai, a postdoc in Yan’s, cultured two separate batches of bacteria — one that produced the adhesion molecules and another that did not — he found that the growth rate of the producers was much lower than the non-producing cheater cells. Clearly, the creation of biofilms was ripe for exploitation. That begged the question, what is it about the environment that has prevented cheater cells from evolving?
To answer that question, the team ran experiments and simulations of Vibrio cholerae cells growing in a biofilm in the laboratory. They found that the further away producer cells were from non-producers, the less able those non-producers were to take advantage of the adhesion molecules secreted by the producers. “The biofilm-creating cells maintain a sort of social distance from the free-loaders,” said Yan.
Next, the researchers used a microfluidics device to send fluids flowing across the biofilm. They also ran simulations to show how the shifts in that flow could be detrimental to the non-producer cells. “It would only be in a completely static environment where there would be a substantial area for cheaters to evolve because in nature these kinds of bacteria are almost always in the presence of flow,” said Tithof.
Those two factors combined – distance and flow – appear to counteract the evolutionary pressure of exploitation.
The researchers believe their study could provide both a conceptual framework and a technical toolset for future research on the population dynamics of bacterial communities. In addition, Yan said that new insights into the adhesion molecules that hold biofilms together could lead to a variety of valuable biomedical and industrial applications.