Microorganisms, though individually capable of significant metabolic feats, often cooperate with other organisms to perform population-wide tasks. One example is the emission of light by biological organisms, termed bioluminescence, a phenomenon in which a population of bacteria must all express their genes encoding for luciferase, a light-producing protein, at the same time. This process is regulated by quorum sensing, the central focus of a 2012 study relating the expression of bioluminescence in Vibrio harveyi to the concentrations of extracellular signaling compounds.
At a basic level, quorum sensing is a type of chemical communication through which individual cells can detect their own cell density. Many types of bacteria secrete small organic compounds called autoinducers (AI) into their environment, which neighboring cells can recognize and uptake. A higher concentration of AI, therefore, corresponds to a more dense accumulation of cells. Once the AIs reach a particular “threshold concentration” a response is triggered simultaneously throughout entire population, such as the induction of bioluminescence in V. harveyi.
In nature, however, quorum sensing proves a bit more complex. Imagine a single cell of V. harveyi that secretes autoinducers into its environment at a constant rate, whether that environment is a dense, exponentially growing population or a completely uninhabited area of water. The secretion of AIs, like any other cellular process, comes at an energy cost to the bacterium. In both of these environments, the cell is investing the same amount of its energy in producing AIs, but the investment is more worthwhile in the more crowded setting—here, the cell is helping the population reach its threshold concentration and achieve a widespread response. In the isolated setting, the cell is expending lots of energy but can’t reach a threshold concentration alone, and runs a greater risk of dying off. As such, species like V. harveyi secrete relatively low levels of AI and, upon detecting enough AI from neighboring cells (indicating that enough neighbors are present), dramatically increase their production.
In this study, researchers determined that V. harveyi secretes three different AI relating to bioluminescence—AI-2, HAI-1, and CAI-1—the concentrations of which fluctuate throughout the population’s growth curve. AI-2 concentration was found to grow rapidly during the population’s exponential growth phase but slow before entering stationary phase, the phase in which cells are no longer growing but not dying off yet. HAI-1 and CAI-1 were both at immeasurably low concentrations until the late exponential phase and stationary phase, respectively, but eventually equaled AI-2 in overall concentration. While each AI is recognized by a different sensor kinase (a protein receptor located in the cell membrane), all activate the same response pathway inside the cell, and are considered to be integrated into a single “signaling cascade.” Each of the three AIs correlated with increased amounts of transcription of the luxR gene that encodes a “master regulator” protein, LuxR, which controls many of the genes involved in bioluminescence. As such, cells in a common chemical environment were able to synchronize their expression of these genes. This study raises numerous intriguing questions about the details of quorum sensing. For example, why does V. harveyi (and many other species) produce three different compounds that elicit the same cellular response, rather than just one? A possibility raised in this article is that different AIs can act synergistically, working together to produce a more intense level of luminescence—providing yet another layer of cooperation to this complex and fascinating process. These potential interactions, and their relationship to the regulation of bioluminescence, are likely topics of future study in quorum sensing.