The Electric Border Collie
Using electric fields, NASA-supported researchers are learning to herd microbes for the benefit of astronauts and Homeland Security.
May 14, 2004: For thousands of years, humans have been herding. Goats. Sheep. Cattle. It's not always easy, and modern ranchers usually have help. They use a Border Collie to keep their beasts together. These dogs are amazing; if they can see it, they can herd it.
But what do you do if you want to herd, say, microbes?
It's not as silly as it sounds. Onboard a spaceship, for instance, a few microbes floating in the ship's drinking supply could be a harbinger of trouble to come. The same is true of urban water supplies. What if terrorists dump pathogens into a city reservoir? Herding microbes together for testing and eradication could save the day.
Right: Giardia lamblia. This single-celled organism makes people sick when it gets into their drinking water. Photo credit: H. Lindquist, US EPA. [More]
NASA-sponsored researchers at Texas A&M University are working on a prototype device, a sort of electric Border Collie, that might be able to herd microbes. The principle is simple: the cell membranes of some pathogens are negatively charged. Electric fields, therefore, could be used to corral the tiny beasts.
Conventional microbe detectors work with very small volumes of water, usually between 10 and 50 millionths of a liter. That can be a problem: If dangerous microbes are widely dispersed in the water supply--as they might be in the early stages of infestation--the odds of finding microbes in such a tiny sample are poor. Choosing a sample to test that contains the harmful microbe is a hit-or-miss proposition.
Pillai and and his collaborator, Texas A&M engineering professor Ali Beskok, have recently received a grant from NASA's Office of Biological and Physical Research to solve this problem.
The device Pillai and Beskok are designing for NASA will use positively-charged electrodes to attract bacteria. The membrane "bag" that separates a bacterium's innards from the outside world is made from a kind of fat molecule. Embedded in this membrane are a wide variety of larger, carbohydrate and protein molecules that control the microbe's interaction with the outside world. These embedded molecules each have a distinctive pattern of positively and negatively charged regions on their surfaces. For the pH levels (i.e., acidity) typical of drinking water, the net charge from these embedded molecules is usually slightly negative, so they will be drawn to a positively charged electrode.
Above: Cartoon of a typical cell membrane. [More]
Their device would channel about 5 liters of recycled water per hour through hundreds of parallel tubes, each only about a quarter of a millimeter wide. The small size of these tubes ensures that any microbes present in the water will pass close by the positive electrode lining the tube wall and become stuck to it. Occasionally, the electrode is switched off and the collected microbes are flushed out into a second, similar device, which concentrates them further. After one more volume-reducing step, most of the bacteria and viruses from dozens of liters of water will be corralled into about a milliliter of water -- a thimbleful -- which can then be automatically divided up and tested to see if dangerous species are present.
"Once you can get a large volume of samples into a small volume, then detecting organisms in that small volume is a breeze," Pillai says.
The device ought to be able to catch more than 90 percent of the microbes that pass through it, Pillai says. Currently he and Beskok are doing research to aid in finalizing the design -- checking to see how strong the electrode must be, for example, and what the optimum width and number of the micro-tubes would be. He hopes to have a prototype ready for testing about a year from now.
Right: Simulated microbes (synthetic microspheres) carrying a fluorescent dye are attracted to positively charged electrodes. Note the plus (+) and minus (-) signs denoting the charge of the electrodes.
"We also got funding from the State of Texas to develop a similar device for testing drinking water supplies," Pillai says. "The water utility of the city of El Paso (Texas) said that they would allow us to field test our device on their distribution system and see if it really works."
"Another version of our device that would be smaller and lighter will be designed for use in space."
It could come in handy onboard the International Space Station, for instance. And certainly such devices will be needed for extended trips to the Moon or Mars. Far from Earth, a Mars-ship's water supply will be continuously recycled, gathering water from every possible source--even an astronaut's exhaled breath and urine. Herding microbes in such an environment is obviously a good idea.
With an electric Border Collie onboard, astronauts can corral microbes and deal with them, before their numbers get out of hand.