Flows that help bacteria feed and organize biofilms

Under threat of being scrubbed away with disinfectant, individual bacteria can improve their odds of survival by joining together to form colonies, called biofilms. What Arnold Mathijssen, postdoctoral fellow in bioengineering at Stanford University, wanted to understand was how stationary biofilms find food once they've devoured nearby nutrients.

Leading an international team of researchers in creating simulations of how fluids move, Mathijssen found that individual bacteria and biofilms can generate currents strong enough to draw distant nutrients.

When bacteria move, they disturb the liquids that surround them in the microscopic world. The researchers explored the strength of that disturbance in a single bacterium that moves in a way that is similar to many pathogenic species, including those that cause gastritis and cholera. They found that as this bacterium swims forward, it creates a tiny but stable current in the surrounding liquid with fluid moving toward its center and away from the head and tail.

Then, they calculated the flows produced by a colony of randomly arranged bacteria and were surprised to see that it created a strong, consistent tide capable of pulling in nutrients. This occurred regardless of the orientation of each bacterium so long as the colony was thicker in some areas than others, which causes fluid to move from high points to low points. Simulations of more orderly bacteria resulted in even stronger circulation.

Within organized biofilms, the researchers found two common patterns of movement: vortexes and asters. In a vortex pattern, the bacteria move in concentric circles and produce a flow that brings nutrients down to the biofilm's center and then pushes the fluid out the sides. In an aster pattern, the bacteria move toward a central point, creating a flow that moves from the edge of the biofilm until it rises back up, over the center.

"The powerful thing about this is you can add these patterns up," Mathijssen said. "Rather than having to know the position and orientation of every single bacterium, you only need to know the basic patterns that make up the colony and then it's very easy to derive the overall transport flow."

The researchers were able to combine vortex and aster patterns within a single biofilm to determine how the bacteria would push, pull and whirl the fluids around them. As a final test, the researchers took calculations representing the complex, realistic motion of bacteria swarming -- as they might on the surface of a table -- and predicted the strength of that swarm's transport flow. The result were large vortices that spanned distances beyond the boundaries of the biofilm, suitable for keeping the colony fed.

See:

Arnold J. T. M. Mathijssen, Francisca Guzmán-Lastra, Andreas Kaiser, Hartmut Löwen. Nutrient Transport Driven by Microbial Active Carpets. Physical Review Letters, 2018; 121 (24) DOI: 10.1103/PhysRevLett.121.248101

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology