Vibrio harveyi is a free-living, gram negative bacterium. Vibrio harveyi are capable of causing disease in fish and invertebrates. This particular bacterium is also the cause for the Milky Sea Phenomenon. The Milky Sea Phenomenon is when there are huge areas of bioluminescent or light-emitting bacteria in the water. It is hypothesized that colonies of Vibrio harveyi produce bioluminescence when there is a microalgal bloom (Phaeocystis) in the surface water. For bioluminescence to occur, very specific conditions need to be met such as high cell density. The required cell density is ~108 cells/mL. This is because Vibrio harveyi, as well as all other bacteria, use quorum sensing.
Quorum sensing is cell-to-cell communication in bacteria that allows them to monitor their population density. During this process, bacteria can “count” their neighbors. This allows control of gene expression throughout the whole bacterial population. In order to count their neighbors, bacteria produce signaling molecules called autoinducers, which are released into their environment. At low cell density, autoinducer concentrations are low and are not easily noticed by their neighbors. This is why high cell density is important. At high cell density, the concentration of autoinducers becomes much greater. This allows bacteria to detect the presence of their neighbors. There are three major quorum-sensing systems; Vibrio harveyi uses the LuxIR system. Basically, once the autoinducer reaches its maximum concentration when the bacteria are at high cell density, it is bound to the protein LuxR. From here, there is a series of phosphorylation cascade events. The end product is an enzyme, luciferase, which is necessary for light production (bioluminescence).
Current research of this process is being used to develop ways to interfere or inhibit quorum sensing. This could be used to kill bacteria instead of using antibiotics, which can lead to drug-resistance.
-by Erin Kawamoto
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