Quorum sensing is a phenomenon defined as gene regulation in response to cell density. The accumulation of 'signaling' molecules in the surrounding environment enable a single cell to sense the number of bacteria (cell density), so that the population as a whole can make a coordinated response. This often leads to auto induction of the signal, and so a rapid increase of signal concentration in the surrounding environment is observed. At critical cell densities, the binding of a regulator protein to the signal leads to the switch on of genes controlled by quorum sensing and, therefore, a coordinated population response. Quorum sensing regulates various functions in multiple bacterial species. In this book the role of Quorum sensing has been elucidated.