"Forget invisible ink or lemon juice – spies can now send messages hidden in genetically engineered bacteria. The new method, dubbed steganography by printed arrays of microbes (SPAM), uses a collection of "Escherichia coli" strains modified with fluorescent proteins that glow in a range of seven colours.
Each character of the message is encoded using two colours, creating 49 possible combinations – enough for the alphabet, the figures 0 to 9 and a few other symbols. "You can think of all sorts of secret spy applications," saysDavid Walt, a chemist at Tufts University in Medford, Massachusetts, who led the research.
The new technique is not the first example of biological encryption – researchers have previously hidden messages in DNA
– but Walt says his method is easier to use. "If you're out in the field and trying to send a message, you're not going to have access to a DNA synthesiser," he says, whereas you could carry vials of bacteria.
The bacterial code has a much lower information density than DNA, however, which would limit the size of a message. "You probably could send 500 to 1000 symbols on a regular piece of paper," says Walt – enough for a quick mission update, but it probably won't let you smuggle state secrets out of a country."