Bacteria on a chip

There’s an old science joke that goes, “If it stinks, it’s chemistry, if it’s green and slimy, it’s biology, and if it doesn’t work, it’s physics.”

Now, however, scientists are messing with these once-sacred boundaries, as they attempt to combine living cells and computer chips to create tiny, inexpensive pollution detectors.

Many cells contain mechanisms designed to protect them from toxins. The presence of a toxin activates a “promoter gene,” which in turn activates other genes that cause the cell to begin producing proteins to help it adapt.

However, it is possible to genetically engineer cells in which the promoter gene instead activates artificially inserted “reporter genes,” which cause the cell to instead produce proteins that create a signal detectable by our instruments. The most commonly used reporter genes are those that code for bioluminescent proteins, such as the ones that give fireflies and jellyfish their glows. The result is a cell, a “bioreporter,” that literally lights up in the presence of a specific toxin.

In a test carried out by the Center for Environmental Biotechnology at the University of Tennessee Knoxville, groundwater at a research facility in Mississippi was deliberately contaminated with simulated jet fuel. The usual way to test for such contamination would be to take samples from wells and send them to an off-site laboratory for detailed analysis, a process involving bulky equipment, a trained technician, hazardous chemicals, and lots of time. In this test, bioreporters designed to glow in the presence of two of the substances in the simulated fuel were combined with the samples and allowed to incubate, then their bioluminescence was measured using a portable photomultiplier interfaced with a laptop computer. (Control samples were sent off-site for the usual analysis.) The bioreporters accurately indicated the presence of the contaminants. Although they didn’t give a very accurate reading on the level of contamination, they allowed researchers to quickly determine the overall movement of contaminants within the groundwater within a few hours and at one tenth the usual cost.

Until recently, the only instruments sensitive enough to detect faint light produced by bioreporters were tabletop devices. Then hand-held, battery-powered units were developed. And now the focus is on Bioluminescent Bioreporter Integrated Circuits (BBICs), which combine bioreporters, photodetectors for capturing faint light signals, signal processors for managing and storing data, and even wireless transmitters for sending the data to a central computer for analysis.

The University of Tennessee researchers who carried out the groundwater test I described are working just such a device for NASA, who are looking for a small, lightweight, low-maintenance method of detecting pollutants in the closed environments of spaceships and space station, which are full of plastics and chemicals and subject to mold and bacterial growth.

Led by microbiologist Gary Sayler, the team has created chips only two millimeters square that combine bioreporters with microluminometers to measure their glow. The whole pollution-sensing device, complete with power source, will probably only be about the size of a matchbox.

The biggest challenge seems to be finding just the right substance to bind the microbes to the chip. It must be transparent, so the bioluminescence can be seen and porous, so any contamination can get to the microbes. It has to provide nutrients for the microbes to feed on, and room for them to grow–but not too much, because an increase in microbes complicates the issue of how much light corresponds to how much contaminant. (Each chip needs a few thousand microbes–just enough to cover the tip of a pin–to generate enough light to be detected.)

The chips will not only tell astronauts when their spaceship is becoming contaminated, they’ll also monitor their own health, and send a signal whenever they need maintenance. The chips won’t have to be replaced, just re-seeded with freeze-dried microbes and moistened, perhaps once every six months or so.

Obviously, there are many Earth-bound uses for such low-cost, low-maintenance detectors. They could detect chemical build-up in your home from new carpets or furniture, for example, or help you track down molds. They could provide early warning of biological or chemical attacks in public spaces. They could even serve as a diagnostic tool for doctors–and implantable bioreporter chip could monitor blood glucose levels in a diabetic and pass the information on to an automated insulin delivery system, for instance. Other chips could scan body fluids for proteins that signal the presence of various diseases, such as cancer.

Chemistry, biology and physics, all tied up in one neat little package that is neither stinky, slimy nor non-working.

Way to ruin a perfectly good joke, guys.

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