Scientists are tweaking bacteria to send encrypted messages that can be shipped via snail mail on sheets of paper-like material called nitrocellulose.

The recipient grows the bacteria with a select cocktail of nutrients and other chemicals. Once grown, each microbe glows one of seven colors when exposed to the right kind of light. Different colored microbes are arranged to represent different letters and symbols. If you know the nutrient and chemical cocktail as well as the keys to the code, you can decipher the message.