Before you ask, SPAM neither refers to the military rations served in Vietnam nor does it apply to the questionable business proposals you receive in your inbox. Instead, it's an acronym for 'steganography by printed arrays of microbes,' a procedure that allows two people to clandestinely communicate using bacteria!
Researchers at Tufts University have perfected a novel way to relay and exchange information using a florescent strain of E. coli. These intestinal flora contain enough color variation to program for all 26 alphabet characters, the numbers 0 through 9 as well as a handful of common symbols. Once encoded, the Escherichia coli are transferred onto thin film which can be mailed or handed off to another secret agent! Finally, the communique is revealed when the bacteria are grown in a fresh Petri dish. It may be extraneous, but SPAM is potentially the most foolproof and secure form of communication we have available.