A Different Kind of Secret Code

Researchers have invented a new form of secret messaging using bacteria that make glowing proteins only under certain conditions. In addition to being useful to spies, the new technique could also allow companies to encode secret identifiers into crops, seeds, or other living commodities. 

The new glowing bacteria actually did grow out of a bit of cloak-and-dagger thinking. Several years ago, the Defense Advanced Research Projects Agency asked researchers to submit ideas for ways to encode secret messages without the need for electronics. At the time David Walt, a chemist at Tufts University in Medford, Massachusetts, teamed up with his former adviser George Whitesides, a chemist at Harvard University. Together, they came up with a way to add a variety of metal salts to a fuse that, when lit, would give off a sequence of pulses of infrared light that encoded a message. That got them thinking about other ways to accomplish the same thing. And so last year they decided to try something else, using bacteria to encode their secrets. 

The new scheme replaces the fuse with seven colonies of Escherichia coli bacteria, each given a gene for a different fluorescent protein. When, and only when, these genes are turned on do the bacteria make these proteins and light up. The colors, including yellow, green, and red, vary based on which gene is expressed. All are clearly visibly different to the naked eye. With their colorful bacterial colonies in hand, the researchers then created a code using pairs of different colored bacteria. Having seven colors gave them 49 combinations, which they used to encode the 26 different letters and 23 alphanumeric symbols such as "@" and "$." They wrote a message by simply blotting pairs of colored bacteria in rows. To "print" the message, the researchers transferred the bacteria onto a plate containing agar, a bacterial growth medium, into which they pressed a sheet of nitrocellulose "paper" that immobilizes the bacteria. 

At this point, the bacteria on the nitrocellulose paper remain invisible. But the message receiver can turn on the key genes and make the colors light up by pressing the nitrocellulose paper into an agar plate containing a chemical trigger that activates expression of the fluorescent proteins. (The proteins chosen to light up are ones the bacteria don't normally use, so unless the researchers activate them, they stay quiescent.) As long as the receiver knows which colors correspond to which characters, the message is revealed. But Walt and his colleagues added one more safeguard as well. Into some bacteria they inserted genes for resistance to particular antibiotics; the idea is that only the antibiotic-resistant bacteria are carrying the real message. If the message fell into the wrong hands, the receiver would see a mix of colors once the genes were activated and be unable to read it. But if the decoder added the right antibiotic, nonresistant bacteria and their colors die away, and the message becomes clear. The first example, reported in today's issue of the Proceedings of the National Academy of Sciences reads "this is a bioencoded message from the walt lab @ tufts university 2010." 

"It's a really cool idea," says Kenneth Suslick, a chemist at the University of Illinois, Urbana-Champaign. So cool in fact that much the same idea was suggested in Spiral, a science-fiction thriller published earlier this year by Cornell University physicist Paul McEuen. In the book, Liam Connor, an aging fungal biologist with enemies, encodes clues to solving a decades-old mystery by inserting genes for fluorescent proteins in the DNA of various fungi. Walt says he hadn't heard of the book, though as a science-fiction fan he's eager to read it. Now with the help of his glowing bacteria, he can write a few mysteries of his own.