Last year, we looked at an idea for a bacteria-powered motor dreamt up by Luca Angelani and pals from the University of Rome in Italy. Their idea was to place a cog with asymmetric teeth into a bath of moving bacteria and wait for them to start it spinning for you, like carthorses pushing a millstone.

We said at the time that the idea sounds a bit like extracting kinetic energy from the random motion of particles, which is impossible because the motion is symmetric in time.

But Angelani and co say there is in important difference between Brownian and bacterial motion: the former is in equilibrium but the latter is an open system with a net income of energy provided by nutrients. This breaks the time symmetry allowing energy to be extracted in the form of directed motion.

Now Angelani and co have built one these asymmetric and persuaded a bath full of E. Coli to push it round at a of 1rpm. Interestingly, Angelani and co report that most of the work is done by just a few bacteria, saying that only 2 out of 10 bacteria attached to a single tooth seem to be contributing to the torque.

In theory, they could speed up the rotation rate by persuading the others to put their backs into it. The linear motion of the gears is currently about 2 micrometres per second while the maximum speed of the bacteria is about 20 micrometers per second. So room for improvement there.

As for applications, they’re limited only by your imagination. Angelani and co envisage self-propelling micro-machines, micropumps and mixers for microfluidics but they also ask whether the idea could be scaled up to power macrmachines. Bacteria–powered cars? Now there’s an idea.

Ref: arxiv.org/abs/0910.2899: A Bacterial Ratchet Motor