Bacteria: the ultimate secret agent

Forget invisible ink, because spies now have a new way to send secret messages to each other: fluorescent bacteria. A team of US chemists has come up with a way to encode messages into arrays of such bacteria, which they call steganography by printed arrays of microbes (SPAM).

David Walt, at Tufts University in Massachusetts, and colleagues created genetically engineered strains of the bacterium Escherichia coli, with each strain able to fluoresce one of seven different colours. To produce messages with these fluorescent bacteria, the group simply deposit individual strains into the wells of a microtitre plate in sets of two, with each pair representing a specific letter, number or punctuation mark. More...

http://www.rsc.org/chemistryworld/News/2011/September/27091102.asp