Encoding Secret Messages in Invisible Bacterial Ink

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Fluorescence images of BL21(DE3)pLysE fluorescent strains after growth and induced FP expression by IPTG.

What kid hasn’t fantasized about sending secret coded messages in invisible ink? Now a team of researchers from Tufts University has developed just such an encryption method using genetically engineered strains of E. coli bacteria.

The researchers designed each of seven strains of bacteria to produce a fluorescent protein of a different color, viewable only under the right wavelength of light. The scientists then created a key of bacteria pairs, with each pair representing one letter, symbol or number. Yellow and green meant H, for instance, and green and orange made I.

The bacteria were then arranged in a grid pattern; when matured, they appeared as spots of color — somewhat like a Lite-Brite toy — spelling out a secret message. The researchers call the messages SPAM, for steganography by printed arrays of microbes, and they envision the information-coding system to be used in forgery-proof watermarks or as an anti-counterfeiting measure — or to send messages between spies.

“Obviously, the secret agent kind of application jumps out,” chemist David Walt of Tufts University, who developed the system with Tufts chemist Manuel Palacios, told Wired. “Somebody embedded in an environment where they need to get a message out but don’t want to be caught.”

Creating each message begins by seeding the bacteria in coded patterns on a growth medium like agar. Once the bacteria mature, the agar plate is pressed against a paper-like sheet of nitrocellulose, transferring the coded bacterial message. That sheet, containing the invisible message, can then be delivered to the intended recipient.

On the receiving end, the nitrocellulose sheet is pressed against another plate of agar and allowed to mature. Using a cipher sent under separate cover, which tells what wavelength of light to use, the recipient can view and decode the message.

The security of the message could be reinforced further by tagging specific kinds of genes with fluorescence. As Wired‘s Brandon Keim reported:

Walt’s group added fluorescence to antibiotic resistance genes, so the message only became apparent when the agar plate was dosed with ampicillin. Most any gene involved in responding to a stimuli — extreme cold or heat, for example, or other nutrients and compounds — could be used the same way, said Walt.

It would also be possible to use E. coli engineered to lose their fluorescence properties over time. “These mutants would add an inherent security measure by self-deleting the message as it develops,” wrote Walt’s team, “similar to the way the Mission Impossible recording self-destructed.”

More likely, SPAM could be used to prevent counterfeiting of, say, high-priced pharmaceuticals, whose adulteration or misdosing could lead to great harm to patients. “You could put the cipher code in an undiscoverable location on the packaging, and the array itself in a protected part of the casing,” Walt told the BBC. “That way, the receiver can authenticate a delivery to make sure it hasn’t been tampered with.”

The study appears in the journal PNAS.

Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.