Yoke Up Those Bacteria
My, how history repeats itself – often in unexpected ways. In ancient times, our ancestors got the heavy work done by hitching oxen, horses or slaves (like Samson, see pictures 1 and 2) to a harness and making them turn a grinding wheel. The same principle is now on the cutting edge of modern applied biological engineering – only now, the movement is measured in micrometers, and the beasts of burden are bacteria. Scientists in Japan, publishing in PNAS,1 have successfully hitched their harnesses to multi-legged crawlers named Mycoplasma mobile and made them turn a gear 20 micrometers wide, many times their size.
In the contraption rigged by Hiratsuka et al., the bacteria walk inside a circular track, pushing a six-petal rotor made of silicon dioxide above them. The inventors (slavedrivers?) developed a surface that would ensure the majority of the cell “microtransporters” would move in one direction with the right amount of friction. The cooperative workers achieved forces of 2 to 5 x 10-16 newton-meters, with rotation rates of 1.5 to 2.6 rpm. “To the best of our knowledge,” they boasted with some merit, “a micromechanical device that integrates inorganic materials with living bacteria has not succeeded until this study.” (They did, however, reference the PNAS research reported in our 08/19/2005 entry, “Saddle Up Your Algae.”)
The inventors didn’t mention evolution once in their paper. Instead, they spoke in glowing terms about their little microscopic oxen and marveled at their technology. First, they scanned the arena of biological micro-machinery with the delight of a gadget freak:
Nature provides numerous examples of nanometer-scale molecular machines. In particular, motor proteins, which efficiently convert chemical energy into mechanical work, are fascinating examples of functional nanodevices derived from living systems. The molecular mechanism underlying the function of these motors has long been a major focus of biophysical research, and the information emerging from those studies should greatly aid in the design and fabrication of novel synthetic micro/nanomotors….
Turning an eye to higher-order biological structures reveals many examples of excellent mechanical devices, including bacterial and eukaryotic flagella and muscle sarcomeres. These motile units are tens of nanometers to several micrometers in size and consist of multiprotein complexes built up with atomic accuracy through the self-assembly and self-organization of protein molecules within cells. In general, these devices work far more efficiently and intelligently than the isolated proteins but, because the principles and mechanisms of self-assembly are only vaguely understood, we are currently unable to assemble higher order motile units from the isolated component proteins outside the cells. Consequently, research aimed at developing hybrid devices using biological motile units is rare at present.
How about the machines employed by their chosen beast of burden? The praise service continues:
Mycoplasma mobile, a species of gliding bacteria, is another example of a higher-order unit (cells in this case) with superb motility. M. mobile has a pear-shaped cell body ~ 1 micrometer in length and moves continuously over solid surfaces at speeds up to 2-5 micrometers per second. The mechanism by which it glides remains unknown, although a mechanical walking model that makes use of the rod-like structures protruding from the cell surface has been proposed. Although three proteins have been identified as essential for gliding, we speculate that this motile system may need a dozen additional proteins, including various cytoskeletal proteins.
So why reinvent the wheel? Why go to all the trouble to invent walking nanorobots, when bacteria have it all figured out? The inventors list other reasons for enlisting biological beasts of burden instead of trying to start from scratch:
As a result, it is currently impractical, if not impossible, to reconstitute fully functional motile units from the isolated proteins of M. mobile in vitro. For that reason, we have been attempting to construct micromechanical devices using intact M. mobile cells instead of the isolated proteins. A key benefit of this approach is that hybrid devices into which living cells are integrated enable us to take advantage of preassembled excellent motor units that have the potential for self-repair or self-reproduction when damaged.
So there you go: spare parts and repairs come included with the package. Oxen must be fed, however, and they didn’t talk about that (cf. Solomon). Someone else may have to invent the nanomanger.
1Hiratsuka et al., “Applied Biological Sciences: A microrotary motor powered by bacteria,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0604122103, published online before print September 1, 2006.
This is nothing less than intelligent-design research in action: intelligent humans seeking to understand and incorporate the built-in technology of intelligently-designed biological machines. The overlap between artificial and biological design is almost seamless. What dividing line could separate the two domains? Who could claim that man’s part represents intelligent design, but the biological part is the result of a long series of mistakes? The researchers even spoke of manipulating the bacteria through genetic engineering to conform the protein moving parts to their design goals. That will blur the distinction even further.
If Martians found our rovers and put them on a track to drive a grinding mill, could they claim with any sense of justice that the rovers must have emerged out of the dust and sand by some unguided process of self-organization? The engineers at JPL would justly be offended. The future belongs to those who think intelligently, and know design when they see it.