Small RNAs in Bacterial Cell-Cell Communication

M any types of free-living bacteria communicate with one another by chemical signals, thereby coordinating metabolic activities and developmental processes. For instance, starved cells of the myxobacterium Myxococcus xanthus release the A signal, a mixture of six amino acids, to promote aggregation, the first step in the development of a fruiting body. Escherichia coli cells coordinate their movement on semisolid surfaces by excreting and sensing aspartate. However, other forms of bacterial communication convey a simpler message: namely, that population densities are at a critical, or “quorum,” level and therefore that particular genes need to be induced or repressed. Quorum sensing depends on signal molecules that bacteria produce in response to increasing cell numbers. Above a critical threshold, those external signal molecules regulate diverse metabolic and developmental functions via intracellular signal transduction pathways.