by Helena Massana-Cid
The celebrity among bacteria Escherichia coli, as many other swimming cells, inspects its surroundings by moving in zigzag motion while searching for "greener pastures." Thus, if a group is swimming in a homogeneous area they will fill all available space uniformly. For many reasons both fundamental and applied, we wanted to understand how to make cells travel to a specific place and then trap them there. So, we wondered: can we be bacteria sheepdogs? Can we round up thousands of them as if they were micro-sheep?
In a recent paper, we show how with computer-generated light patterns we can guide bacteria into a small, confined bacteria sheepfold (Figure 1). To turn bacteria into obedient sheep, we genetically modified them to produce a protein called proteorhodopsin so they would respond to light stimuli. In our strains, proteorhodopsin functions as a light-driven proton pump. Thus, the bacteria use light energy to power flagellar motors, as if it had a nano solar panel, which results in bacteria moving faster in areas with brighter light and slower where it is dark. We demonstrate that with a computer-controlled light projector we can generate thousands of light specks to illuminate the individual cells that are moving towards a desired area and contain them there, as shown in this video. Bacteria that try to escape will find themselves in the dark, swim much slower and will consequently be unable to travel far from the herd and eventually change direction to be illuminated and guided towards the rest of the group. This projected light pattern, the "optical feedback," changes dynamically and is calculated by manipulating with geometric transformations of a microscope image of the bacteria. This approach is simple and scalable to large collections of cells. Specifically, we project into the sample a dilated and distorted image of their past configuration after a fixed time delay. In this way bacteria are directed towards the confined herd. In the paper we also explain how to further exploit this method to move confined clouds of bacteria from one place to another and merge them or split them on-command (Figure 2).
Swimming bacteria, specifically E. coli, are excellent biological machines that synthetic biology techniques now allow us to program at will. On top of that, this study provides a strategy for moving bodies at the microscale, which can give further insight on the out-of-equilibrium dynamics of living active matter, an ongoing challenge in physics and biology, and break ground for light-manipulated trapping and transport of particles within miniaturized laboratories.
Helena Massana-Cid is a postdoctoral researcher in the group led by Roberto Di Leonardo at Sapienza University of Rome, where she studies the physics of light-guided bacteria as part of the SYGMA project, funded by the European Research Council. In 2019 she obtained her Ph.D. at the University of Barcelona, supervised by Pietro Tierno, focusing on the out-of-equilibrium dynamics of active and magnetic colloids.