N-acyl homoserinelactone-mediated gene regulation in gram-negative bacteria.
The view of bacteria as unicellular organisms has strong roots in the tradition of culturing bacteria in liquid media. However, in nature microbial activity is mainly associated with surfaces where bacteria form highly structured and cooperative consortia which are commonly referred to as biofilms. The ability of bacteria to organize structurally and to distribute metabolic activities between the different members of the consortium demands a high degree of coordinated cell-cell interaction. Recent work has established that many bacteria employ sophisticated intercellular communication systems that rely on small signal molecules to control the expression of multiple target genes. In Gram-negative bacteria, the most intensively investigated signal molecules are N-acyl-L-homoserine lactones (AHLs), which are utilized by the bacteria to monitor their own population densities in a process known as 'quorum sensing'. These density-dependent regulatory systems rely on two proteins, an AHL synthase, usually a member of the LuxI family of proteins, and an AHL receptor protein belonging to the LuxR family of transcriptional regulators. At low population densities cells produce a basal level of AHL via the activity of an AHL synthase. As the cell density increases, AHL accumulates in the growth medium. On reaching a critical threshold concentration, the AHL molecule binds to its cognate receptor which in turn leads to the induction/repression of AHL-regulated genes. To date, AHL-dependent quorum sensing circuits have been identified in a wide range of gram-negative bacteria where they regulate various functions including bioluminescence, plasmid conjugal transfer, biofilm formation, motility, antibiotic biosynthesis, and the production of virulence factors in plant and animal pathogens. Moreover, AHL signal molecules appear to play important roles in the ecology of complex consortia as they allow bacterial populations to interact with each other as well as with their eukaryotic hosts.