Prokaryotic Intercellular Signalling

Abstract

Although most bacteria are single-cell creatures, they are far from solitary, independent organisms. Indeed, many bacteria in the natural environment are in close physical proximity in dense populations where they are involved in a desperate competition for food and space. In these environments, intercellular signalling is common and perhaps unavoidable. Although we have only begun to understand the range of interactions affected by signalling, the diversity of those few systems currently being explored is striking. Cell signalling can involve soluble signal molecules such as pheromones which diffuse between cells, cell-associated signals which require intimate contact, or both. Cell-to-cell communication via diffusible signals often takes place at high cell densities, as part of an adaptive response to impending nutrient depletion and crowding. Thus far, the majority of cell-associated signals have been identified in microbes that exhibit complex structural development patterns. However, as new approaches to identify cell bound signals are developed, the list of signalling mechanisms that require cell-to-cell contact is likely to increase. For both diffusible and cell-contact-dependent signalling, high population densities are often required. The various niches where one finds the highest concentrations of bacteria are the surfaces of tissues of fungi, plants and animals, as well as the surfaces of soil particles and other inanimate objects where nutrients may be concentrated. In this Chapter we will attempt to encapsulate the current understanding of prokaryotic intercellular communication, focussing first on the better understood diffusible signalling pathways, and following with a discussion of cell-associated mechanisms.