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Letter Bugs

Invisible ink made from germs? Seeing is believing

Author JACQUELINE DETWILER

MEDFORD, MASS.

When designing experiments, scientists don’t usually consider what kids might like for their birthday, but it seems Manuel Palacios and David Walt of Tufts University have arrived at the perfect present (were it at all appropriate for children). Forget the Acme spy kit — these researchers have discovered how to make invisible ink out of germs. Using bacteria that they genetically modified to light up in different colors, they built a simple code (e.g., red plus green equals the letter “m”) and laid out the bacteria in order on a special piece of paper. The technology could be used for secret watermarks to, say, protect shipments of high-value pharmaceutical drugs from thieves. Here’s how they did it.

1. Specialty research suppliers sell genes for jellyfish proteins that glow blue, green, red, orange or yellow when exposed to fluorescent light. Palacios and Walt ordered a bunch and attached them to a common bacteria, then tested them to see which were the brightest.

2. The scientists used the bacteria to lay out a color-coded message on “paper” made of nitrocellulose. When the message’s intended receivers got it, all they had to do was press it into bacteria food (no, not your son’s socks — it’s called agar) to make it grow. After two days, they shined a fluorescent light on it and the colors appeared.

3. To better hide the secret missives, Palacios and Walt made some bacteria antibiotic-resistant. To read the message, the receivers grew it, then doused it in ampicillin before shining the light on it. The ampicillin killed off all the extra glowing bacteria, leaving only the message behind.

ILLUSTRATION BY DAN MATUTINA

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