Many bacteria like E. coli are fantastic swimmers moving by more than ten times their length in a second, roughly the same speed (in body lengths per second) of a running cheetah. To do that they use a rotary motor, the flagellar motor, that can spin thin helical propellers at more than a hundred turns in a second. The flagellar motor is a kind of electric motor, powered by a flux of electric charges that the cell constantly accumulates in the periplasmic space surrounding the cell inner membrane.

The mechanism by which bacteria “recharge their batteries” usually requires oxygen and is called respiration. In 2000 a new protein, called proteorhodhopsin, was discovered by sequencing genomes of bacterioplankton from the ocean. Proteorhodhopsin sits on the cell membrane and can use energy from light to efficiently replace aerobic respiration and keep the batteries charged even in the absence of oxygen.

At the University Sapienza of Rome, a team of researchers from the Physics Department and the Institute for Nanotechnology of the CNR, lead by physics professor Roberto Di Leonardo, has shown that genetically modified bacteria, expressing the protein proteorhodhopsin, can be used as tiny propellers in micromachines that are invisible to the human eye and whose speed can be reliably and continuously tuned by shining green light of controlled intensity.

“Using short laser pulses for 3D nano-printing”, says G. Vizsnyiczai, “we could fabricate 3D micromotors, whose torque generating units are  circular rings carrying an array of microchambers, each one capable of trapping an individual swimming cell and slave it to the rotor."  By coupling a light projector to the microscope, the researchers were capable of shining spotlights of controllable brightness on each individual rotor and fine tune rotation speeds by an automatic feedback loop.

“Our design combines a high rotational speed with an enormous reduction in fluctuations when compared to previous attempts based on wild-type bacteria and flat structures”, says R. Di Leonardo, “we can be produced large arrays of independently controlled rotors, that use light as the ultimate energy source and which could serve one day as cheap and disposable actuators in microrobots collecting and sorting individual cells inside miniaturised biomedical laboratories”.

This research, published on Nature Communications, has been funded by the European Research Council  within project SMART “Statistical Mechanics of Active Matter” (PI R. Di Leonardo).