Chemical communication--do we have a quorum?

There are two types of bacterial communication systems, those in which the signal produced by bacteria is directed only at other organisms, and those where the signal is detected by others and self. The latter is involved in adaptation to the environment. The adaptation signals are autoinducers, the response is population density-dependent and has been termed "quorum sensing". Our current knowledge of bacterial signaling systems indicates that Gram positive bacteria use small peptides for both types of signaling, whereas Gram negative organisms use homoserine lactones as autoinducers. Gram- negative bacteria internalize the signals which act upon an intracellular receptor. Gram-positive bacteria use the signals as ligands for an extracellular receptor of a two-component signaling system. Inhibitors of quorum sensing compounds for both Gram positive and Gram negative bacteria are being explored. Signal inhibitors could be potentially effective in impeding biofilm formation, which might prolong the utility of the currently available antibiotics in this era of antibiotic resistance. In this review, we will explore both bacteria-host and bacteria-bacteria communication systems, with an emphasis on inhibitors of these systems both natural and synthetic.