ABSTRACT
It is now known that co-ordination of action is not restricted to cells constituting multi-cellular organisms. Several unicellular organisms use sophisticated signaling mechanisms to estimate the concentration of other cells belonging to their own species in their surroundings. The assessment of local population density occurs through secretion and detection of small signaling molecules, which are often specifi c for that species. Once the population is estimated to have reached a threshold, the unicellular organisms collectively initiate a change in their gene-expression profi les, resulting in co-ordinated forays into activities that would not have been profi table if commenced at a lower cell count. Assessing local cell density and employing it to change their actions allows unicellular organisms to function as multi-cellular systems. These cell-to-cell communication systems are collectively known as quorum sensing (QS). QS in various species regulates diverse activities including formation of biofi lms, initiation of virulence, competence, mating, sporulation, formation of root nodules, synthesis of secondary metabolites and bioluminescence (Bassler and Losick 2006). Although QS has also been reported in unicellular eukaryotes, in this chapter we will focus on bacterial QS and their applications in biotechnology (Kruppa 2009).
