Bacteria Navigate Terrain by Exploiting Fluid Landscapes Around Yeast

In the world of microorganisms, microbes compete for turf, spew chemicals at foes, and sometimes exploit the microscopic terrain to gain an edge. In a study published June 4 in the Cell Press journal Biophysical Journal, researchers found that bacteria can speed up by using the fluid pockets shaped by neighboring yeast cells. These microscopic moisture trails allow bacteria to swim farther and spread faster-revealing a new way for microbes to travel through soil, plants, and the human body. 

When studying microbial interactions, research often focuses on the chemical nature of these interactions. But we learned that physical properties also play an important role in how microbes grow and spread." 

Divakar Badal, lead author of Cornell University

The researchers focused on Pseudomonas aeruginosa, a bacterium with tail-like propellers that thrives in soil and human airways, and Cryptococcus neoformans, a stationary yeast. Under a microscope, the researchers watched as the two species closed in on each other and the bacteria then swarmed into the puddle-like fluid surrounding the yeast. They found that bacteria cultured with yeast spread up to 14.5 times faster than when cultured alone and that isolated bacterial colonies quickly connected into continuous clumps. 

At a microscopic scale, Pseudomonas aeruginosa is comparable to a grain of rice in comparison to the yeast, which is about the size of a grape in context. These larger bodies draw moisture from the surface, forming a thin halo of fluid that acts as a temporary swimming lane. This allows the bacteria to bypass the usual limits of a dry surface. Replacing live yeast with dead ones, or even glass beads, produced the same effect, indicating that the behavior was driven by the puddles. 

"The bigger the obstacle, yeast and glass beads alike, the more fluid you have around it, and it's better for Pseudomonas," says Varsha Singh, co-senior author of the University of Dundee. "So, it's leveraging what could have been an obstacle to move farther ahead." 

The researchers also found that the spread of the bacteria ebbs and flows with the landscape created by growing yeast cells. To better understand these dynamics, they built a model to simulate the interactions between the two species. The model showed that faster-growing yeast species like Candida albicans altered the fluid landscape more dramatically, affecting how quickly bacteria could travel. 

"I was absolutely blown away by how well our model predictions match the experimental results," says Danny Raj M, co-senior author of the Indian Institute of Technology Madras. "In a sense, the model is a virtual lab that simulates real behaviors. By changing the parameters, from growth rates to humidity, we can answer a number of questions." 

The implications of this work extend beyond the model and the lab, says the team. In nature, bacteria and yeast coexist in soil, water, plants, and the human body. The ability to ride fluid films may help bacteria colonize these environments more effectively, especially when moisture is scarce. Next, the team plans to examine the species' interactions in the real world. 

"We tend to think of microbiology in an anthropomorphic way, focused on human lungs or the gut because we can relate to them," says Singh. "But much of it plays out in the soil and other environments. That gives us a wonderful opportunity to explore new questions. I think that's where the next frontier is."