Researchers have created a kind of "invisible ink for the biotech age" by spreading bacteria — engineered to glow — in encrypted patterns on paper, reports Wired's Brandon Keim. Tufts University chemist David Walt, who co-developed the system, says this "secret agent" application could be used to send cryptic messages. Walt's team reported its work recently in PNAS. The researchers' system, called InfoBiology, builds on earlier work done by various other researchers, and involves putting codes in DNA and synthesizing it to make certain combinations of bases correspond to certain letters, Keim says. The researchers searched for a way to synthesize different-colored bacteria to correspond to the different letters, and decided to use fluorescent proteins. "Fluorescent proteins, which glow under ultraviolet light and are produced when a selected gene becomes active, are ubiquitous in genetic research, where they’re used to monitor gene activity," Keim says. "Walt's team designed seven strains of E. coli, each a different color. For that bacterial library of seven characters, a simple cipher was generated: One green unit and one orange, for example, would equal an 'I,' while a red and green together meant 'S.'" The bacteria were then seeded on an agar plate, and once they grew, were pressed onto a paper-like nitrocellulose sheet, which became imprinted with the same pattern. Viewed under a fluorescent light, the message comes through loud and clear, Keim says, and can be decoded by anyone with a cipher.