Secret codes in the form of arrays of bacteria:
http://news.sciencemag.org/sciencenow/2011/09/a-different-kind-of-secret...
http://www.pnas.org/content/early/2011/09/19/1109554108.abstract
I especially liked how they made them reveal their information only under some conditions, and that confounding non-message bacteria were mixed in.
They have past publications leading up to this,
http://www.pnas.org/content/106/23/9147
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