Munching Microbes Sleuth Out Hidden Pollutants

In the land of biscuits and sausage gravy, scientists are adding an "hors d'oeuvre" to the menu of methods for detecting tainted soil.

Dubbed "critters on a chip," the rudimentary device uses genetically engineered microbes nestled on a small computer chip to signal the presence of pollutants.

The tiny device holds the promise not only of dramatically cutting the cost of detecting and monitoring soil and water pollution, its developers say. It also could be tailored to help detect chemical and biological warfare agents, as well as explosives. "Imagine what might have been avoided if this technology had been installed in the Tokyo subway stations" at the time of the terrorist attack that gassed several underground stations in 1995, says Mike Simpson, a research engineer at the Oak Ridge National Laboratory in Oak Ridge, Tenn., and one of the developers of the device.

The chip is a byproduct of the first-ever field test of bacteria genetically engineered to clean up contaminated soil, according to Gary Sayler, a microbiologist and director of the Center for Environmental Technology at the University of Tennessee at Knoxville.

The notion of "redesigning" some of nature's microscopic minions to clean polluted soil and water has intrigued researchers for years. Compared with the high cost of stripping tainted soil off the land and treating it, allowing cheaply replicated microbes to do the work on the spot will be a bargain, they say - especially if the polluted soil lies buried deep beneath the surface.

Researchers have been working in the lab to engineer microbes to break down a range of pollutants since the late 1980s, but safety concerns about outside-the-lab releases made outdoor testing difficult, Dr. Sayler explains. Subsequent research has answered those concerns.

After getting a thumbs-up from the US Environmental Protection Agency, Sayler and colleagues at Oak Ridge began the open-air experiment last autumn on a test plot laced with naphthalene, one of a class of chemicals known as polyaromatic hydrocarbons.

These chemicals, a byproduct of burning or processing fossil fuels, "are a big worldwide problem," Sayler says. "They come out in ashes, waste oils, and as sludge. You'll find these in land fills or waste sites that in some cases are more than 100 years old."

To monitor the process, Sayler and his colleagues engineered the bacteria, whose formal name is Pseudomonas flourescens HK44, to emit light as they digested naphthalene.

They used genes from bacteria in blue-green algae responsible for the algae's glow. In essence, glowing HK44 are munching away and otherwise satisfied with the their general living conditions.

"We started looking for other ways to use and improve the organisms," Sayler says, and up came the notion of using them for environmental monitoring.

Sayler teamed up with Dr. Simpson, a research engineer at the Department of Energy's Oak Ridge National Laboratory. Simpson, who designs photodetecting chips for physics experiments, explains that one of silicon's properties is its ability to convert light into an electric current. He placed from 10,000 to 100,000 of the engineered bacteria on a thin layer, or substrate, of glass atop a specially designed 2-millimeter-by-2-millimeter chip. When the bacteria were exposed to naphthalene, they glowed, and the chip picked up the signal.

The key, he says, "is to work with the surface of the chip to make it friendly" to the bacteria culture that will do the glowing. One problem with the prototype, he adds, is that as the bacteria process the naphthalene, they use the glass as one source of oxygen - in essence they eat their way through the glass. The next step, he continues, is to find a less scrumptious substrate, perhaps silicon nitrate.

Sayler adds that "with micro and nanoscale techniques, you could envision making a chip that incorporates a microscale test tube" with pores large enough to allow chemicals to seep in but small enough to prevent the bacteria from escaping. The chips could send their signals to instruments above the soil either through wires or by incorporating tiny radio transmitters into the chip's design.

Other researchers have been looking at ways to combine enzymes and computer chips to make chemical sensors. Enzymes are proteins that act as catalysts for chemical reactions in living organisms.

Yet, says Sayler, outside the protective shelter of a cell wall, enzymes quickly degrade. One advantage of using an entire organism, such as a bacterium, he adds, is that its cell wall protects the biological compounds doing the detecting.

Moreover, by genetically tailoring bacteria to digest different chemicals, it would be possible to design a single chip sensitive to several hazardous compounds.

Although any commercial application for the chip may be as much as 10 years away, Simpson says that more sophisticated versions could be ready for trial applications within a year or two.