Using electric fields, researchers
are learning to herd microbes for the benefit of astronauts and
by Patrick L Barry and Dr Tony
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.
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 and
run to the rescue! The principle is simple: the cell membranes of
some pathogens are negatively charged. Electric fields, therefore,
could be used to corral the tiny beasts.
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.
"The biggest roadblock
for any agency - whether it's Homeland Security or for NASA or for
EPA or anybody else - is to monitor a large quantity of water [for
small numbers of microbes]," notes Suresh Pillai of Texas A&M.
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.
Cartoon of a typical cell membrane
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.
"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.
microbes (synthetic microspheres) carrying a fluorescent
dye are attracted to positively charged electrodes. Note
the plus (+) and minus (-) signs denoting the charge of
"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.