In its simplest form, quorum sensing refers to the ability of bacteria to communicate. It describes the capacity of certain bacteria to exhibit coordinated behavior in response to a particular population density. These bacteria usually rely on the production, accumulation, and subsequent response to diffusible signal molecules in order to sense population. These signal molecules accumulate in environments that can sustain a sufficiently dense population, or quorum, of the signal-producing bacteria. When the concentration of the signal molecule reaches a critical level, the quorum-sensing bacterial population responds through the concerted expression of specific target genes.