Andrew Dopheide has created an animation that illustrates signaling and quorum sensing.
"A solitary bacterium cannot form a biofilm by itself - it must wait until a group of bacteria has gathered. With no fingers to count on, how do bacteria know when there are enough others nearby? Bacteria are equipped with molecular signalling systems. Individual bacteria steadily emit signal molecules, which can be detected by other nearby bacteria. These signals allow bacteria to determine what other cells are out there and what they are doing, and are used to coordinate processes such as biofilm formation. This process is known as quorum sensing.
As bacteria increase in abundance, signals will be detected at higher frequency. When the signal frequency reaches a certain threshold, this may trigger a change in gene regulation and reconfiguration of cellular machinery for a new purpose, such as production of pili and extracellular polymers."