No, not licking them.
Colour-changing frog cells could be used to detect performance-enhancing drugs and leave cheating athletes red-faced. Sensors based on the cells can pick up traces of drugs in body fluids, and could even detect new drugs that other methods fail to pick up.
Some frogs, such as the African clawed frog (Xenopus laevis), change colour to cope with sunlight and heat and also to improve their camouflage. They do this by activating cells in their skin that contain granules of melanin, the dark brown pigment.
These colour-changing cells, called melanophores, are normally dark but can be triggered by a particular hormone released in the frog. When the hormone binds to the cell wall, it sets off a reaction that moves the pigment granules to the centre of the cell, making it look colourless.
Once the hormone detaches, the melanin grains disperse throughout the cell, making it appear dark again. This colour switch has now been adapted to detect tiny amounts of opiates - including heroin, morphine and codeine.
Annika Karlsson and her colleagues at Linköping University in Sweden zapped melanophores with an electric field to tear tiny holes in their membranes, and then inserted strands of human DNA into the cells. The DNA then churns out cell-membrane receptors that opiates will bind to. The cells were cultured and transferred to small plates.
To check whether activating these receptors could trigger the colour change, Karlsson added drops of naloxone, a drug that blocks opiate receptors by binding strongly to them. Naloxone is used in rehabilitation programmes because it reduces the effects of drugs like heroin and morphine.
Karlsson found that the cells lightened significantly when the naloxone was added. "The response was obvious, even with the naked eye," she says. Although the team didn't test the precise sensitivity of the cells, she says that tests worked using just tens of micrograms of naloxone per litre.
The advantage of the frog-based sensor is that it doesn't need to know what drug to look for, says Hagan Bayley, head of medical biochemistry and genetics at Texas A&M University in College Station.
"You could have someone who's taken something that activates the opiate receptor and you'd detect it without even knowing what the drug is," he says. So if an athlete was using a very new opiate-type drug, the test would pick it up.
The sensor has other uses too. Since it changes colour with any chemical that acts like an opiate, it could be used to screen chemical compounds for useful opiate drugs, says Bayley.