Microbes use quorum sensing to coordinate certain behaviors based on the local density of the bacterial population. Microbes that use quorum sensing constantly produce and secrete certain signaling molecules (called autoinducers or pheromones). These microbes have a receptor that can specifically detect the signaling molecule (inducer). When the inducer binds the receptor, it activates the transcription of certain genes, including those for inducer synthesis. As the microbial population grows the concentration of the inducer passes a threshold, causing more inducers to be synthesized. This forms a positive feedback loop, and the receptor becomes fully activated.
Activation of the receptor induces the upregulation of other specific genes, causing all of the cells to begin transcription at approximately the same time. This coordinated behavior of microbial cells can be useful in a variety of situations such as multiplying.
Read more – http://en.wikipedia.org/wiki/Quorum_sensing