May 11, 2010 | David F. Coppedge

Bacteria: Let’s Harness Those “Perfect Machines”

Ten Italian scientists have a novel idea.  They want to hitch up their wagons to bacteria and use them to power nanomachines.  It’s too much work to build such “perfect machines” from scratch, they said.  Why not just take advantage of what nature has already provided?
    Their paper in PNAS1 is downright dreamy about the possibilities of using flagella-equipped bacteria as motors for their machinery:

Self-propelling bacteria are a nanotechnology dream.  These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns.  Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices.  Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively.  Here we show that asymmetric environments can produce a spontaneous and unidirectional rotation of nanofabricated objects immersed in an active bacterial bath.  The propulsion mechanism is provided by the self-assembly of motile Escherichia coli cells along the rotor boundaries.  Our results highlight the technological implications of active matter’s ability to overcome the restrictions imposed by the second law of thermodynamics on equilibrium passive fluids.

That’s an interesting term for a living cell – “active matter.”  From an engineer’s viewpoint, a bacterium can be considered a little bit of energetic matter that has figured out how to overcome the second law of thermodynamics, at least locally and temporarily.  They do it by converting biochemical energy into mechanical motion.  All engineers need to figure out is how to apply this active matter in asymmetric ways so that work can be done at the nanometer scale with a kind of “Brownian ratchet” effect (see 04/19/2010).
    The team ran some proof-of-concept experiments with tiny gears immersed in an active bath of motile E. coli cells.  They got their bacterial helpers to turn the gear at 2 rpm.  In contrast to previous attempts that tried to channel the bacteria along guided tracks (09/06/2006), this team relied on a self-organizational method.  “We demonstrate that the underlying off-equilibrium nature of a bacterial bath allows one to rectify the chaotic motions of bacteria by geometry alone,” they said, implying that the pre-imposed design in the setup brings the order out of the chaos.  “Once the microstructures are fabricated with a proper asymmetric shape, no further chemical patterning or externally induced taxis is needed to produce a directional and predictable motion.”  A dream of great physicists may soon become a reality, thanks to our little cellular friends:

The idea that, in nonequilibrium states, a directional motion can arise from the chaotic dynamics of small molecules was first put forward by [Richard] Feynman in his famous “ratchet and pawl” thought experiment [1966].  The combination of asymmetry and nonequilibrium was soon recognized to be at the origin of the “ratchet effect,” opening the way to the stimulating concept of Brownian motors in physical and biological contexts.  Many ways have been considered to drive a system out of thermal equilibrium, such as cycling temperature or applying time-dependent external fields.  Our experiment demonstrates an intriguing realization of a ratchet mechanism, where bacteria can be thought of as intrinsically off-equilibrium “molecules.”  Asymmetric environments can be used to break the remaining spatial symmetries and allow the emergence of an ordered, reproducible motion that could serve as the driving mechanism for completely autonomous, self-propelling microdevices.  Applications at the micrometer scale, such as self-propelling micromachines or pumps and mixers for microfluidics, are the most promising, but it will also be important to answer the question whether bacterial motors are confined to the microworld, or we can think of a macroscopic exploitation of bacteria as mechanical power sources.

Now that’s a dream; imagine bacteria as power sources for our automobiles.  How many flagella would that take?  The paper was edited by Howard Berg of Harvard, one of the pioneer investigators of the bacterial flagellum.  The authors made no mention of evolution.

1.  Di Leonardo et al, “Bacterial ratchet motors,” Proceedings of the National Academy of Sciences published online before print May 10, 2010, doi: 10.1073/pnas.0910426107.

Evolution?  They had no need of that hypothesis.  We hate to keep rubbing this in, Eugenie, Ken, Richard, Jerry, and Francisco, but the first step in recovery is to admit you have a problem.  We know it will get depressing when nobody calls wanting to hear your rendition of the Theodosius Soliloquy,* and the universities can’t attract bright students fast enough to enter their biomimetics labs.  But you can still have a future in the Design Age.  Find a hobby; plant tomatoes, try art, take a hike.  Life will go on.

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