Apparently, bacteria use at least two methods to communicate, using plasmids for intra-species communication, and autoinducers for interspecies communication. I wonder if this communication could (or already has) become complex enough to constitute a global microbial mind?
'Rosetta Stone' Of Bacterial Communication Discovered (http://www.sciencedaily.com/releases/2009/07/090707093619.htm)
The Bacteria Whisperer
(http://www.wired.com/wired/archive/11.04/quorum.html)
Bacteria Communication: Counting Heads or Measuring Space? (http://www.sciencedaily.com/releases/2007/04/070402103058.htm)
Quorum-sensing: The phenomenon of microbial communication (http://www.iisc.ernet.in/currsci/mar102006/666.pdf)
Decoding Microbial Chatter: Cell-Cell Communication in Bacteria (http://http://jb.asm.org/cgi/content/full/187/16/5507)
Interactions in Bacteria (http://euarch.blogspot.com/2007/10/interactions-in-bacteria.html)