As implied in the term single-cell organisms, bacteria used to be regarded as lonely individual cells that act independently from their neighbouring cells. Research in the past four decades has painted a completely different picture. Bacteria survive and thrive in communities in every imaginable habitat. In each community, bacteria communicate with each other and with other species to coordinate functions that are difficult or impossible to achieve by individual cells. These include uptake and processing of nutrients, coping with environmental stresses, and mounting attacks on host organisms. A ubiquitous bacterial communication strategy is quorum sensing, whereby bacterial cells sense and respond to changes in their local densities by the production and sensing of small, diffusible molecules. Bonnie L Bassler and E Peter Greenberg elucidated many of the molecular mechanisms underlying quorum sensing as well as the implications of the mechanism in controlling bacterial physiology in the context of infectious diseases. Understanding quorum sensing is of fundamental significance for explaining how bacteria interact with each other or with their physical environment. It points to innovative ways to interfere with bacterial pathogens or to modulate the microbiome for health applications, and establishes a technological foundation for precisely controlling bacterial dynamics using artificial gene circuits.
The recognition of quorum sensing and the elucidation of its underlying mechanism are one of the most fascinating developments in microbiology. The notion of bacterial cells communicating within and between species has transformed the way we think of bacteria or interpret the implications of gene regulatory mechanisms. While numerous quorum sensing systems have been discovered, they share the same fundamental architecture. Each cell produces a small molecule that is released into the environment by diffusion or excretion. The concentration of the molecule then reflects the density of the producing cells and can trigger gene expression in cells able to respond to this molecule, through a cognate receptor protein. This incredibly simple yet elegant mechanism enables bacteria to sense changes in their local densities or the physical confinement, and to coordinate behaviour within a population or between populations of the same or different species. It plays a critical role in controlling diverse functions, including generation of bioluminescence, formation of biofilms, and development of virulence. In addition to their roles in bacterial physiology, the molecular components underlying quorum sensing have been widely used in synthetic gene circuits to program dynamics of one or multiple bacterial populations in time and space.