Chemicals are used to turn the bacteria on and off. They push their bead forward at speeds of around 15 microns per second. Sitti and Behkam began by sticking several S. marcescens – the kind of bacteria that cause pink stains on shower curtains – onto polystyrene beads 10 microns in diameter. These tiny “robots” were suspended in a solution containing water and glucose. To stop the bacteria’s motion, the researchers add copper sulphate to the solution. The copper ions bond to the rotor of the flagella motor and prevent it from moving. To restart the motion, another chemical called ethylenediaminetetraacetic acid (EDTA) is added. The EDTA traps the copper ions attached to the rotor, allowing it to move again. The rotors can be switched off in this way an unlimited number of times. The bacteria themselves are only about one-fifth of the size of each bead and adhere to them via electrostatic, van der Waals forces and hydrophobic interactions. As the attached bacteria rotate their flagella, feeding on surrounding glucose.
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