The term 'quorum sensing' describes intercellular bacterial communication which 
regulates bacterial gene expression according to population cell density. 
Bacteria produce and secrete small molecules, named autoinducers, into the 
intercellular space. The concentration of these molecules increases as a 
function of population cell density. Once the concentration of the stimulatory 
threshold is reached, alteration in gene expression occurs. Gram-positive and 
Gram-negative bacteria possess different types of quorum sensing systems. 
Canonical LuxI/R-type/acyl homoserine lactone mediated quorum sensing system is 
the best studied quorum sensing circuit and is described in Gram-negative 
bacteria which employ it for inter-species communication mostly. Gram-positive 
bacteria possess a peptide-mediated quorum sensing system. Bacteria can 
communicate within their own species (intra-species) but also between species 
(inter-species), for which they employ an autoinducer-2 quorum sensing system 
which is called the universal language of the bacteria. Periodontal pathogenic 
bacteria possess AI-2 quorum sensing systems. It is known that they use it for 
regulation of biofilm formation, iron uptake, stress response and virulence 
factor expression. A better understanding of bacterial communication mechanisms 
will allow the targeting of quorum sensing with quorum sensing inhibitors to 
prevent and control disease.