Quorum sensing is a regulatory system used by bacteria for controlling gene expression in response to increasing cell density. This regulatory process manifests itself with a variety of phenotypes including biofilm formation and virulence factor production. Coordinated gene expression is achieved by the production, release, and detection of small diffusible signal molecules called autoinducers. The N-acylated homoserine lactones (AHLs) comprise one such class of autoinducers, each of which generally consists of a fatty acid coupled with homoserine lactone (HSL). Regulation of bacterial quorum sensing signaling systems to inhibit pathogenesis represents a new approach to antimicrobial therapy in the treatment of infectious diseases. AHLs vary in acyl group length (C4-C18), in the substitution of C3 (hydrogen, hydroxyl, or oxo group), and in the presence or absence of one or more carbon-carbon double bonds in the fatty acid chain. These differences confer signal specificity through the affinity of transcriptional regulators of the LuxR family. C16-HSL is one of a number of lipophilic, long acyl side-chain bearing AHLs, including its monounsaturated analog C16:1-(L)-HSL, produced by the LuxI AHL synthase homolog SinI involved in quorum-sensing signaling in S. meliloti, a nitrogen-fixing bacterial symbiont of certain legumes. C16-HSL is the most abundant AHL produced by the proteobacterium R. capsulatus and activates genetic exchange between R. capsulatus cells. N-Hexadecanoyl-L-homoserine lactone and other hydrophobic AHLs tend to localize in relatively lipophilic cellular environments of bacteria and cannot diffuse freely through the cell membrane. The long-chain N-acylhomoserine lactones may be exported from cells by efflux pumps or may be transported between communicating cells by way of extracellular outer membrane vesicles.
