Many motile bacteria, such as Escherichia coli, continuously explore the surrounding space in search of the best growing conditions.

As grazing animals, if left in an open area, bacteria spread evenly over 'meadows' wherever food is available. Rounding them up is a very challenging task, especially when they are numerous and run fast.

Researchers of the Department of Physics of Sapienza University of Rome have demonstrated that microscopic spots of light, like thousands of sheep dogs, can herd even the fastest bacteria into a confined area. This is only possible if the bacteria are genetically modified to produce proteorodopsin. This proton pump uses light energy like a mini solar panel to move the flagella (long, thin cell appendages with a motor function). The study, published in Nature Communications, is the result of Sapienza University's collaboration with CNR-Nanotec and the Italian Institute of Technology.

"These bacteria," says Helena Massana-Cid, a researcher at Sapienza's Department of Physics and the first author of the study, "move faster when the light is intense and more slowly in dark areas. Therefore, a computer-controlled light projector consisting of tiny reflectors pointed at individual cells was used to round them up, and it quickly switched off the light on the bacteria that were trying to escape from the area".

Using a digital camera linked to the microscope, the researchers obtained images of the bacterial suspensions, which were processed in real time through geometric transformations and then projected onto the sample with a fixed time delay. Thus, moving illuminated by this distorted image of their past, thousands of bacteria can move together, like a herd, towards a specific region.

Particles capable of consuming energy to move actively, such as motile bacteria, are part of a broad class of non-equilibrium systems, both synthetic and biological, collectively called 'active matter'. Although predicting and controlling the behaviour of these systems is still an open challenge between physics and biology, this experiment opens the way for interesting developments in both fundamental aspects of non-equilibrium physics and its applications.

"At a fundamental level," concludes Roberto Di Leonardo of the Department of Physics of Sapienza and corresponding author, "we were able to establish a mathematical relationship between the geometric properties of the projected light patterns and how bacteria respond by distributing themselves in space. As for future applications, light could be used to trap and transport clouds of active particles in miniaturised laboratories, using mechanical energy to drive micro-machines with both biological and synthetic components".

References:

Rectification and confinement of photokinetic bacteria in an optical feedback loop – Helena Massana-Cid, Claudio Maggi, Giacomo Frangipane, Roberto Di Leonardo – Nature Communications (2022) https://doi.org/10.1038/s41467-022-30201-1

Further Information

Roberto Di Leonardo
Department of Physics
roberto.dileonardo@uniroma1.it