Acyl side chains of these signals can be fully saturated, they can have hydroxyls or carbonyls on the third carbon, and they can have lengths of 4 to 16 carbons (ref. The acyl side-chain length and the substitutions on the side chain provide signal specificity. aeruginosa LasI directs the synthesis of N-(3-oxododecanoyl)-HSL (3OC12-HSL). Thus Pseudomonas aeruginosa RhlI primarily catalyzes the synthesis of N-butyryl-HSL (C4-HSL), and P. Different LuxI homologs generate different acyl-HSLs. The enzyme is generally a member of the LuxI family of acyl-HSL synthases. In most described cases, acyl-HSL signals are generated by the activity of a single enzyme that uses as substrates S-adenosylmethionine and an intermediate of fatty acid biosynthesis, acyl-acyl carrier protein ( 11– 15). Because acyl-HSL signaling provides a mechanism by which a bacterial species can monitor its own population density, this type of signaling and other signaling systems that achieve the same purpose have been termed quorum-sensing systems ( 10). These chemical signals are produced by specific enzymes, and they are detected by specific receptors. Acyl-HSLs are small molecule signals with no other known function. As will be discussed below, acyl-HSL signaling represents a dedicated communication system that is used by bacteria to control specific genes in response to population density. The signaling pathway is an enticing target for the development of antipathogenic therapies. This type of bacterial cell-to-cell communication was discovered in the context of microbial ecology, but it is now evident that acyl-HSL signaling is important in plant and animal (including human) diseases. Perhaps the best-studied signaling system is the acyl-homoserine lactone (acyl-HSL) system used by a large number of Gram-negative bacterial species. Many Gram-positive bacteria use small peptides in signaling one another ( 4, 5), Gram-negative bacteria appear to use small molecule signals of various sorts ( 1, 6– 9). Several different types of signals have now been described.

It is now clear that many bacterial species use chemicals to signal each other and to coordinate their activities. This concept has given way to the view that, like other creatures, bacteria can communicate with each other and form communities that represent more than the sum of the individuals (for example, see refs. After all, dense cultures of bacteria can be grown from a single cell. It was once held that most bacteria function only as individuals designed to compete with one another and to multiply rapidly under appropriate conditions. There are multiple quorum-sensing circuits that control the expression of dozens of specific genes that represent potential virulence loci. aeruginosa model, quorum sensing is required for normal biofilm maturation and for virulence. Acyl-homoserine lactone signaling in the opportunistic animal and plant pathogen Pseudomonas aeruginosa is a model for the relationships among quorum sensing, pathogenesis, and community behavior. This type of quorum sensing represents a dedicated communication system that enables a given species to sense when it has reached a critical population density in a host, and to respond by activating expression of genes necessary for continued success in the host.

One type of cell–cell signaling is acyl-homoserine lactone quorum sensing in Gram-negative bacteria. Species-specific cell-to-cell communication is involved in successful pathogenic or symbiotic interactions of a variety of bacteria with plant and animal hosts. Recent advances in studies of bacterial gene expression have brought the realization that cell-to-cell communication and community behavior are critical for successful interactions with higher organisms.

Gram negative bacteria