The secret microbial messages are first grown in Petri dishes. The cultures are then transferred to a thin film and ready to be sent to the desired undercover recipient.
To unlock the message, the recipient must transfer the bacteria to a genetically modified growth medium, which acts as the secret key.
For example, the bacteria could be engineered to react only with a certain antibiotic, therefore allowing the message to only be revealed when in contact with that specific chemical.
If any other chemical key is used the message would be scrambled.