It is now well established that a number of bacteria communicate through diffusible signals that may induce and/or regulate a coordinated response by the individual organisms that make up a given population or biofilm. For many of these organisms, it has been suggested that intercellular signaling functions to report population density or to coordinate a response from all cells in a microbial community. Therefore, cell-to-cell communication has been referred to as auto-induction or quorum sensing. The response of bacteria to quorum sensing signals is quite varied and includes, for example, the induction of bioluminescence, the regulation of virulence gene expression, the formation of biofilms, or the induction of horizontal transfer of genetic material. It is also becoming increasingly apparent that some bacteria may communicate via contact-dependent signaling mechanisms, and that the response to direct cell-to-cell contact influences complex behaviors that may contribute to multicellular development or the adaptation to growth in complex biofilms. In the past five to ten years, increased interest and research in the mechanisms of bacterial cell-to-cell communication has revealed surprising complexity both in the signaling processes themselves and in the breadth of the response of recipient cells to the signal molecules. For example, a variety of chemical species, e.g. acyl-homoserine lactones, oligopeptides, furan derivatives (i.e. AI-2), quinolones, butyrolactones, and unsaturated fatty acids are known or have been suggested to function as diffusible signals. Furthermore, some organisms, most notably Pseudomonas aeruginosa and species of Vibrio, have been shown to produce and respond to multiple diffusible signal molecules.