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 bioilluminating 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.
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