Vibrio fisheri are a marine gram negative bacterium and they live symbiotically with marine animals like the squid below. However these glowing bacteria are only found bio illuminating in a large enough colony. How are these bacteria able to sense the number of bacteria around them? With a little bit of regulation and the right genes they can turn on their light like a light bulb wired to a switch. Any genes needed for the bacteria to illuminate are turned off when the densities of cells are low, but in crowded conditions such as within the light organ of the Hawaiian bobtail squid they illuminate.
This acyl homoserine lactone quickly exits the cell and can quickly buildup concentrations outside the cell until it will reenter. Within the cell the LuxI molecule will bind a regulatory element and when bound bio illumination genes are activated, the regulatory molecule for Vibrio fisheri is LuxR. The LuxR complexed with the auto inducer acyl homoserine lactone, Lux I, will navigate to the genome of the cell and bind the lux operon. The complex acts as a transcriptional enhancer by upregulating other lux proteins to form. Two of the genes, luxA and luxB, generate protein products which form a luciferase protein, responsible for light production.
Why is it important to turn the lux system on at certain times? Just as we turn off a light when its not in use the lux system exists to conserve energy. By producing the bioillumination gene products at specific times the bacterium conserves its chemical energy. The Hawaiian Bobtail squid allows large colonies of Vibrio fisheri to occupy its crowded light organ, and the bacteria provide luminescence for the squid after sunset. During the day the bobtailed squid, Euprymna scolopes, hides buried in the sand of shallow reef waiting for the sun to set. In the darkness, the nocturnal animal emerges searching for food. Under a moonlit night, the squid would appear as a dark silhouette when it swims through the water and could easily be detected by predatory fish. However, the squid camouflages itself by projecting light downward to appear as the night sky. The squid swimming in a moonlit night would appear camouflaged to fish below because the squid would match the night sky.
Quorum sensing is simply bacterial chemical communication and allows the bacterial cells to determine the size of their colony. With this information they know when to upregulates other genes as virulent attacks within our bodies. This is why Quorum sensing is an active area of microbiology research. If the quorum sensing process was stopped then deadly bacteria could exist, but remain docile.
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Shadel GS, Baldwin TO. The Vibrio fischeri LuxR protein is capable of bidirectional stimulation of transcription and both positive and negative regulation of the luxR gene. J Bacteriol. 1991 Jan;173(2):568–574
Visick KL, Foster J, Doino J, McFall Ngai M, Ruby EG. 2000. Vibrio fischeri lux genes play an important role in colonization and development of the host light organ. J. Bacteriol. 182:4578–4586.