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," says David Walt, a chemist at Tufts University in Medford, Massachusetts, who led the research.
Messages are grown on agar plates then transferred to a thin film that can be sent in the post to the recipient. The film appears blank in everyday conditions, but the message is revealed when the recipient transfers the bacteria to an appropriate growth medium.
As well as giving the bacteria their fluorescent palette, genetic modification also defines which growth medium they will respond to ? so the medium type can act as a secret key. For example, bacteria engineered with resistance to a certain antibiotic will display a message only when treated with that particular chemical ? any other antibiotic will produce gibberish, or could even display a message warning that the wrong key has been used. Walt says that combining a number of genetic traits could lead to thousands of possible keys.
It is also possible to develop bacteria that lose their fluorescent properties over time, creating a message that self-destructs in the style of Mission Impossible.
Small secrets only
The new technique is not the first example of biological encryption ? researchers have previously
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.
"It's a nice piece of work, but I'm really doubtful about the practical relevance," says Dominik Heider, who researches DNA cryptography at the University of Duisburg-Essen, Germany. He says that simply sending an encrypted email would be a more useful way of transmitting secret messages; the DNA method, for instance, is normally used to watermark genetically modified organisms rather than help spies complete their mission. Heider also points out that many countries place restrictions on sending genetically engineered bacteria through the post.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1109554108
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