Quorum sensing (QS) is a communication mechanism used by bacteria to recognize cell population density fluctuations and control gene expression, which is a critical role both in intra- and interspecies communication and controls microbe-host interactions. QS i s t he process in which the bacterial cells detect threshold concentration of signaling molecules in the external environment, and then after having exceeded this allowable threshold, they respond accordingly and modify their behavior by altering the expression of their genes. Regulation of gene expression in response to the density of bacterial cells in a population is a key phenomenon in the mechanism of QS and it is used by both Gram-negative and Gram-positive bacteria. In Gram-negative bacteria LuxR protein plays a key role in QS system as a type of transcription regulators and participates in a variety of biological behaviors with LuxI protein and signal molecules, including those encoding virulence factors and antibiotics biosynthesis, plasmid transfer, bioluminescence, and biofilm formation. New researches which highlight the unusual signaling molecules, novel regulatory components and heterogeneity in the QS system of Gram-negative bacteria are presented in this paper.