Quorum-sensing in gram-negative bacteria

It has become increasingly and widely recognised that bacteria do not exist as solitary cells, but are colonial organisms that exploit elaborate syste ms of intercellular communication to facilitate their adaptation to changin g environmental conditions. The languages by which bacteria communicate tak e the form of chemical signals, excreted from the cells, which can elicit p rofound physiological changes. Many types of signalling molecules, which re gulate diverse phenotypes across distant genera, have been described. The m ost common signalling molecules found in Gram-negative bacteria are N-acyl derivatives of homoserine lactone (acyl HSLs). Modulation of the physiologi cal processes controlled by acyl HSLs (and, indeed, many of the non-acyl HS L-mediated systems) occurs in a cell density- and growth phase-dependent ma nner. Therefore, the term 'quorum-sensing' has been coined to describe this ability of bacteria to monitor cell density before expressing a phenotype. In this paper, we review the current state of research concerning acyl HSL -mediated quorum-sensing. We also describe two non-acyl HSL-based systems u tilised by the phytopathogens Ralstonia solanacearum and Xanthomonas campes tris. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.