Microfluidic mixology with bacterial stir bars

In microfluidics, fluids often follow a laminar flow that keeps streams separate and prevents mixing. However, mixing reagents can often be advantageous because it speeds up reactions. Gurung et al. explored how a rotating flagellar motor from a rod-shaped bacterium can induce mixing in microfluidic systems.

To create the mixer, the team tethered bacteria to the microfluidic surface with a sticky filament, which is part of the bacterial flagellar motor.

“In nature, these bacteria swim by rotating a propellor to propel through a solution. Here we have ‘stuck down’ the propellor, but the motor still rotates,” said author Matthew Baker. “It would be a bit like grabbing a submarine propellor with a giant set of pliers and watching the whole submarine turn around and around. Likewise, we stick the propellor down and watch the whole cell body go around.”

The researchers remotely activate the rotation with green laser light. Upon illumination, a membrane protein pumps out a proton and creates a gradient. In response, the bacterium tries to maintain proton concentration by rotating its flagellar motor.

“Bacteria have the potential to drive mixing at the micron scale by generating micro vortices like a magnetic stirrer,” said author Jyoti Gurung.

In the future, the authors plan to explore better and different methods to control rotation via light. They are also interested in manipulating the patterning of bacteria to control fluid flows more precisely.

“Our understanding of this motor and our ability to engineer it show promise for controlling mixing – and one day fluid flow directly – when synthetic biology of microbes is combined with microfluidics,” said Baker.

Source: “Microbial stir bars: Light-activated rotation of tethered bacterial cells to enhance mixing in stagnant fluids,” by Jyoti P. Gurung, Moein Navvab Kashani, Charitha M. de Silva, and Matthew A. B. Baker, Biomicrofluidics (2023). The article can be accessed at https://doi.org/10.1063/5.0144934 .