Quorum Sensing Mechanisms in Gram Positive Bacteria

Abstract

Quorum sensing (QS) is a form of intercellular communication that enables bacteria to coordinate gene expression in a density-dependent fashion. Bacterial signaling molecules called autoinducers are central to this process. When released into the surrounding environment they bind to signaling receptors on the surface of neighboring bacteria, and upon reaching a threshold level activate quorum sensing genes. Gram-positive bacteria employ small post-translationally modified peptides called autoinducing peptides (AIPs) as signaling molecules. AIPs are often integral elements of a histidine kinase two-component signal transduction system. In certain cases the secreted AIPs may be imported back into the cell after release. They are then identified by cytoplasmic transcription factors. In this system, extracellular proteases process the secreted precursor-AIP into mature AIP. Upon return to the cell, the mature AIPs bind to, and alter the activity of the corresponding transcription factors. Some examples of such transport are know to be critical in sporulation, competence, and enzyme production in Bacillus subtilis. A large gamut of peptides is secreted from bacteria by de novo biosynthesis and proteolytic degradation. These peptides include pheromones that modulate expression of specific genes of Gram positive bacteria to regulate biosynthesis of quorum dependent proteins such as virulence factors in addition to serving critical roles in a myriad of bacterial life processes such as regulation of the bacterial competence, bacterial conjugation and bacterial virulence. Interestingly, bacterial cells can respond to the AIPs secreted by itself as well. The physiological effects of this ‘self-sensing’ have been studied rather recently in Bacillus subtilis.