Primarily produced by Gram-negative bacteria, acylated homoserine lactones (AHLs) are a class of small neutral lipid molecules composed of a homoserine lactone ring with an acyl chain.   AHLs produced by different species of Gram-negative bacteria vary in the length and composition of the acyl side chain, which often contains 4 to 18 carbon atoms.  AHLs are synthesized by AHL synthases. They diffuse in and out of cells by both passive transport and active transport mechanisms.   Receptors for AHLs include a number of transcriptional regulators called “R proteins,” which function as DNA binding transcription factors or sensor kinases.