Bacterial Integrated Circuits

 Like a canary in a mine, a microbe can often sense environmental dangers before a human can. It's easy to see a canary's reaction. But how can you can you tell what a microbe's feeling? How can you coax a microbe to communicate?

One way is to interface it to a silicon chip.

University of Tennessee microbiologist Gary Sayler and his colleagues have developed a device that uses chips to collect signals from specially altered bacteria. The researchers have already used these devices, known as BBICs, or Bioluminescent Bioreporter Integrated Circuits, to track pollution on earth. Now, with the support of NASA's Office of Biological and Physical Research, they're designing a version for spaceships.

Right: Glowing colonies of microbes. [More]

Sayler's group, which includes Tennessee researchers Steve Ripp, Syed Islam and Ben Blalock, as well as collaborators at JPL and the Kennedy Space Center, has bioengineered microbes that glow blue-green in the presence of contaminants. Then they joined those bacteria to microluminometers--chips designed to measure the light.

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What BBICs offer, explains Sayler, is a low-cost, low-energy way to detect pollutants. They're tiny: each BBIC is about 2 mm by 2 mm, and the entire device, including its power source, will probably be about the size of a matchbox, and they monitor their surroundings continuously.

NASA is interested in sensing contaminants because spaceships are tightly sealed. Unseen fumes from scientific experiments or toxins produced by molds and other biofilms can accumulate and pose a hazard to astronauts. BBICs can be crafted to sense almost anything: ammonia, cadmium, chromate, cobalt, copper, proteins, lead, mercury, PCBs, ultrasound, ultraviolet radiation, zinc--the list goes on and on.

The system is surprisingly rugged. Microbes thrive in a wide range of environments, so it's possible to design BBICs that can survive in extreme or highly contaminated surroundings. "They can actually do their job sitting in things such as jet fuel-water mixtures," marvels Sayler.

Left: The integrated circuit microluminometer. Actual size is 2 mm by 2 mm. [More]

Although the microbes can protect themselves from toxins, they still have a variety of needs--food, for example. Keeping them alive, Sayler says, "is a significant portion of the work."

One problem is that microbes must be immobilized so that they remain right next to the chip. The challenge, says Sayler, is trying to figure out how to immobilize the microbes in such a way that they survive as long as possible.

The researchers are testing various substances that will keep the microbes in place. Something with good optical transparency is critical, of course, so that if the microbes light up, the chip can perceive that. The immobilant has to be porous, so that any contamination can flow in, and reach the microbes. It has to contain nutrients for the microbes to feed on. It has to allow the microbe enough, but not too much, room. "We're basically trying to feed the immobilized organisms in the matrix without them growing. We really don't want them to grow very much, if at all. If they grow, it changes the total amount of cells in the system, and it confounds the issue of how much light corresponds to how much contaminant."

(There needs to be about a few thousand microbes per chip, says Sayler, in order to generate enough light. That's not as many as it seems, though -- it's only about enough to cover the tip of a pin.)

Right: The basic architecture of a BBIC. [More]

Sayler hopes to develop gels in which the microbes can be kept functional for several months. The sensors would probably be attached to the spaceship walls, continuously monitoring the ship's atmosphere. They'd monitor themselves, too, to make sure that the microbes were still viable. "We can electrically induce cells to make light, so we can pulse the system every once in a while to see if the organisms are still physiologically active."

"After, say, six months, the chip would send a signal that says, 'oops, time to replace your bug sensor.' An astronaut would go and get a freeze-dried package of seed microbes, add a little moisture, and stick it in the sensor." Nothing more has to be done until the next time the signal goes off, six months later. It's a low maintenance system.

Below: Gary Sayler is the director of the Center for Environmental Biotechnology at the University of Tennessee, Knoxville. [More] These BBICs are useful on Earth, too. They can detect formaldehyde emitted by pressed wood furniture or hard-to-detect molds often implicated in sick building syndrome. "If this device works as planned, it could turn out to be a very inexpensive kind of monitoring system," says Sayler. "You could go to your corner drugstore, buy one of these, take it home and stick it up on your wall. It could tell you whether your carpets are degassing, or whether you've got problems like black mold."

Advanced BBICs could serve as bioterrorism monitors for Homeland Security, as a means to detect DNA radiation-damage in astronauts, or as a diagnostic tool for doctors. An example: Sayler envisions BBICs as part of a treatment program for diabetics. An implantable BBIC equipped with an on-chip radio transmitter could monitor blood glucose levels and communicate with a remote insulin delivery system. Such devices could also scan body-fluids for certain proteins that signal tumors--in other words, an early warning system for cancer.

Much more research needs to be done before these ideas become reality. Making BBICs work on spaceships is a good place to start.

http://science.nasa.gov