MEET the Siamese fighting fish鈥攖he latest weapon in the battle against terrorism.
Because their skin changes colour in the presence of almost any toxin, it can be used to detect rare biological and chemical weapons that terrorists might use.
In the past, governments thought they knew what biological and chemical weapons threats they faced. But the spectrum of threats from terrorism is much broader. Because the new detection system is based on living cells it will respond to anything that interferes with them, not just the toxins they are programmed to expect. 鈥淚t is a hypersensitive surrogate for a human being,鈥� says Phil McFadden of Oregon State University, the biologist who revealed the idea at a conference in Pittsburgh last week.
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His device contains colour-changing skin cells called chromatophores. The fighting fish use these cells as camouflage and to attract mates. Although they don鈥檛 normally function as toxin detectors, anything that harms them also interferes with the colour-changing mechanism.
The cell鈥檚 hue depends on the position of its pigment granules. These are transported around the cell by motor proteins attached to tiny filaments. If the granules are near the cell surface, the cells are bright, but when they are transported inwards the colour dims. In a living fish, for example, a sex hormone can trigger a chain of reactions that activate the motor proteins and change a cell鈥檚 colour.
Toxins can subvert one or more links in this chain. A toxin binding at an intermediate point in the chain can trigger the same reaction as the sex hormone. Some toxins dim the cells and others brighten them, so the device sounds an alarm either way.
In McFadden鈥檚 detection system, skin cells are bathed in a nutrient medium containing sugars, amino acids and salts that keeps them alive for up to three months. To detect a contaminant, a sample of, say, air or water is drawn in or a piece of suspect soil is dissolved in the nutrient medium. An electronic sensor detects the colour change.
McFadden has tested about 100 environmental pollutants, biological weapons and infective agents on the system, including cholera toxin and Bacillus bacteria鈥攖he genus that includes anthrax. All the species that are toxic to humans set the system off, while harmless species had no effect.
To be of any use, the new system must not cry wolf, says David Franz, a former UN weapons inspector who served in Iraq after the Gulf War. As he puts it: 鈥淗ow many false positives do you want in your subway at rush hour?鈥� So far, the system is looking good: the response from the cells has matched human reactions almost exactly.
Only one type of neurotoxin failed to set the cells off, as it binds to a component of nerve cells that isn鈥檛 present in chromatophores. McFadden suggests a real detection system might contain three or four different cell types to cover everything.
This is not a detection system just for anthrax, he says. If you want to counter a specific threat there are better ways of doing it. But these days, he adds, we have to consider crazy twisted minds that might make weapons out of things we had never thought of before.
