September 6, 2006 | David F. Coppedge

Flagellar Swimmers Attain Mechanical Nirvana

Those little germs that scientists love, E. coli – you know, the ones with the flagella that intelligent-design folk get all excited about – well, they move through the water pretty efficiently with those high-tech outboard motors of theirs.  Some Pennsylvania physicists reporting in PNAS1 measured the “swimming efficiency of bacterium Escherichia coli” and concluded, “The propulsive efficiency, defined as the ratio of the propulsive power output to the rotary power input provided by the motors, is found to be ~ 2%, which is consistent with the efficiency predicted theoretically for a rigid helical coil.”  An engineer can’t get much more efficient than that, in other words, even in theory.  Later in the paper, they summarized, “The measured [epsilon: i.e., propulsive efficiency] is close to the maximum efficiency for the given size of the cell body and the shape of the flagellar bundle.”
    That efficiency rating is the overall measurement for the package.  Many bacteria have multiple flagella, however, and ascertaining the individual contributions of each component, and the subtle hydrodynamic interactions between them, is a difficult task.  They did, however, assess the length of the flagellum as a factor in the optimal performance, and concluded that “flagella are as long as required to maximize its propulsive efficiency.”2
    They measured the swimming efficiency by capturing single bacteria in “optical tweezers” and putting them into a measured rate of flow.  The work was edited by Howard Berg of Harvard, a pioneer of flagellum research (see his 1999 article on Physics Today).


1Chattopadhyay, Moldovan, Yeung and Wu, “Swimming efficiency of bacterium Escherichia coli,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0602043103, published online before print September 5, 2006.
2For a dazzling animation showing how the flagellum tip is constructed, see the video link from our 11/02/2005 entry.  Fast-forward to 18:20.  How does it know when to stop growing?  There must be feedback from the growing tip to the control mechanism in the cell body.

Man-made outboard motors are stubby-shaped, loud, polluting, inefficient monstrosities that generate huge wakes.  Since the bacteria have already mastered propulsive efficiency, maybe this will inspire some boat builder to do a little biomimetics.  We should see if flagellar construction scales up to human proportions and maintains the efficiency rating.  If so, lakefront property owners would love them for it.
    Too bad we can’t ask the little critters how they came up with this technology.  It wouldn’t help anyway, probably.  All they would say is, “Dunno; we’re just the customer.”

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