Quorum sensing in bacteria

Quorum sensing is the regulation of gene expression in response to fluctuat ions in cell-population density. Quorum sensing bacteria produce and releas e chemical signal molecules called autoinducers that increase in concentrat ion as a function of cell density. The detection of a minimal threshold sti mulatory concentration of an autoinducer leads to an alteration in gene exp ression. Gram-positive and Gram-negative bacteria use quorum sensing commun ication circuits to regulate a diverse array of physiological activities. T hese processes include symbiosis, virulence, competence, conjugation, antib iotic production, motility, sporulation, and biofilm formation. In general, Gram-negative bacteria use acylated homoserine lactones as autoinducers, a nd Gram-positive bacteria use processed oligo-peptides to communicate. Rece nt advances in the field indicate that cell-cell communication via autoindu cers occurs both within and between bacterial species. Furthermore, there i s mounting data suggesting that bacterial autoinducers elicit specific resp onses from host organisms. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quor um sensing systems differ, in every case the ability to communicate with on e another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Presumably, this process bestows upo n bacteria some of the qualities of higher organisms. The evolution of quor um sensing systems in bacteria could, therefore, have been one of the early steps in the development of multicellularity.