Quorum Sensing Systems in Aeromonas spp.

Abstract

Quorum sensing (QS) is an intercellular communication system by which bacterial cells are capable of indirectly monitoring their own population density through production and exchange of diffusible signal molecules. This enables bacteria to control gene expression dependent on population size and thereby perform coordinated phenotypic changes in a multicellular fashion. At present, QS regulatory systems have been reported for Gram-positive and Gram-negative bacteria. These systems rely on two major components, a small diffusible signalling molecule which accumulates in a population density-dependent manner and a transcriptional activator protein which, in concert with the signalling molecule, activates the expression of relevant genes. For Gram negatives, they employ N-acyl homoserine lactones (acyl HLs, AHLs) as the signalling molecule. AHL synthase encoded by LuxI homolog synthesizes AHL molecules. Short side-chain AHLs diffuse freely across cell membranes, whereas long side-chain AHLs have to use active efflux to partition to the membrane (Pearson et al. 1999). Upon reaching a threshold concentration in the extracellular medium, AHL molecules are perceived by cytoplasmic LuxR family proteins to regulate the downstream processes (Parsek and Greenberg 2000). As a consequence, expression of certain functions, often virulence factors required for pathogenesis, is triggered at high population densities in a coordinated manner.