Consider the marine propeller: This device for moving vesselsthrough water consists of a rotating screw or paddle attached to adrive-shaft, which turns in a stator, behind which an energy sourceimparts torque to the propeller.

Archimedes, the Greek polymath, reputedly invented the rotatingscrew to bail water out of a large boat. In the 1850s, the BritishAdmiralty divided a 20,000 cash award among five inventors of themodern marine propeller.

In fact, nature did it first, in Escherichia coli, the bacterial workhorseof genetic engineering: This capsule-shaped microbe, a primeinhabitant of every human intestine on earth, measures two micronslong by one micron in diameter _ that is, one one-thousandth of amillimeter by two. Lined up in a row, nose to tail, some 800 copiesof E. coli might span the diameter of this hyphen (-).

But does E. coli have a nose, or are both its ends the same?

Two bacteriologists, John Parkinson and David Blair at theUniversity of Utah, Salt Lake City, wrote a think-piece in Sciencedated March 19, 1993, titled: "Does E. coli have a nose?" Theyraised the question because two Stanford scientists, reported in thesame issue of Science on "Polar location of the chemoreceptorcomplex in the Escherichia coli cell."

Harvard biophysicist Howard Berg didn't believe that E. coli had anose, and set out to prove his doubt. He reports as much in a papertitled "Cells of Escherichia coli swim either end forward," in thecurrent issue of The Proceedings of the National Academy ofSciences (PNAS), dated Jan. 17.

Rotor - Stator - Bushing - Torque, Nano-Scale

The microbes swim through their culture medium propelled by half adozen spinning flagellae. "That's the rotor," Berg told BioWorldToday. There's a stator, a drive-shaft and the bushing that gets thedrive-shaft through the outer end of the cell. All these mechanicalcomponents, he added, "are remarkable if for no other reason thantheir size: The rotor is only about 50 nanometers in diameter." That'sone-twentieth of E. coli's one-micron diameter.

These rotating flagellae control the direction in which the microbeswims in its never-ending search for nutrient sugars and amino acids."A wild-type cell swims in a reasonably straight line for 30 micronsor so, about a second," Berg explained. "Then it jumps about veryerratically, in place, for about one-tenth of a second, before takingoff in a new direction, more or less at random."

Guiding this apparently patternless pattern of motion is chemotaxis,E. coli's built-in radar for sensing the presence and concentration offood. The receptors that respond to changes in the chemicalenvironment _ as the two Stanford authors reported two years agoin Science _ cluster near one end of the cell. It was this finding thatelicited the notion that E. coli has a nose at its front end to sniff outand taste nutrient concentrations.

Berg dismissed this hypothesis because "The diffusion of smallmolecules is so efficient on the scale of an E. coli cell, which is onlya micron, that you can't gain much by putting receptors in the frontas compared to the back. In other words, you do sniff a few moremolecules in front, and fewer in back, but it's only a percent or two,at best."

To test whether E. coli does in fact tend to swim one end forward,Berg, a tenured professor of molecular and cellular biology atHarvard University, began by marking cells with tetrazolium. Thisbright dye collects in a granule at one end or another of the cell. "Wefound," he said, "that you could see these particles in dark-fieldmicroscopy; they look just like automobile head lamps. Recordingwhat the marked cell does by videotape, we could see which end ofthe cell is which, and see what it did."

What it did was swim, or run, in a straight line, stop and tumble, thentake off at a tangent for another spurt of swimming. "Whether yourun or tumble," Berg explained, "depends on the direction that yourflagellar motors are turning. If clockwise _ looking at the propellercoming out of the cell from behind _ the microbe just sits there andtumbles about.

"But if it turns its flagellae counterclockwise, they coalesce into abundle that stands out behind the cell, going around, and that tendsto push the cell forward. It just runs, smoothly.

"When they turn again the other way, the bundle becomes unstableand flies apart. Its several filaments begin working independently,pushing the cell this way and that." Berg continued. "With thattumbling, the cell body is reorienting itself rapidly and erratically,not as a gymnast would do it, but more of a spastic motion."

This behavior is to measure concentrations, and compare what itmeasured over the past second to what it found in the previous threeseconds. "Responding to the difference," Berg said, "it asks whetherthe concentration is going up or down as it swims along. If it's goingup, that reduces the likelihood that it's going to tumble. It tends tokeep its rotors going counterclockwise, so the favorable runs getlonger." And when the gradient of something that it likes to eat isgoing down, then it reverses to its baseline behavior."

Evolution developed this system, Berg continued, "so the cell couldmigrate toward the more fertile region, where there was more to eat.Strong attractants, such as glucose, tend to be good nutrients. Strongrepellents, such as alcohol, tend to be noxious."

Berg also runs a laboratory on bacterial motility and behavior at theRowland Institute for Science, in Cambridge, Mass., near Harvard.He sees his PNAS report as laying to rest the idea of a nose on E.coli's face. This "basic research, not motivated by any practicalutility," he observed, "is a wonderful model for understandingsensory transduction and behavior at the molecular level, and may bean inspiration for other people."

Now his main interest is "trying to understand how the bacterium'spropeller works, how it's built, how you build a machine like that.It's nano-technology in the extreme." n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.

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