Raise your hand if you’ve heard of Aliivibrio fischeri! (Formerly known as Vibrio fischeri.)
A. fischeri are Gram-negative, rod-shaped, motile bacteria with a yellow-ish pigment. They are found in most all marine environments, but aren’t all that plentiful. There’s no evidence that A. fischeri cause disease in humans or other organisms, but it does contain homologs of toxin genes found in the Vibrio species.
What sets Aliivibrio apart from regular-Joe bacteria, however, is its ability to bioluminesce (glow in the dark!) and live symbiotically within various marine organisms.
The Hawaiian bobtail squid is one organism that plays host to A. fischeri. In exchange, the squid use the bacteria’s bioluminescence to attract prey, deter predators, and communicate with other squid.
So, how do the bacteria get into the squid?
First, squid hatchlings secrete a special mucus that attracts A. fischeri into the squid’s light organ. This organ is exposed to seawater, so it’s not hard to attract the right bacteria. Once a decent number of A. fischeri have accumulated in the light organ (around 100 bacteria within 12 hours), the squid stops producing mucus.
These bacteria aren’t permanent residents, however. Each day, the squids expel up to 95% of their light-producing bacteria. It’s up to the remaining bacterial cells to establish a new population within the light organ - this is important, since the bacteria only bioluminesce once their population reaches a certain size.
And how do the bacteria produce light?
The whole process of bioluminescence revolves around the enzyme luciferase. This enzyme, along with other enzymes required for the light reaction, are encoded by the luxCDABE operon.
These enzymes catalyze the oxidation of reduced flavin-mononucleotide and a long-chain fatty acid to produce blue-green light (and other not-so-interesting things). Not only do the bacteria produce luciferase, they also have a fatty-acid reductase that makes the fatty acids required for the light-producing reaction.
And for the chemically-inclined, here’s said light-producing reaction:
FMNH2+O2+R-CHO → FMN + R-COOH + H2O + light
Free-living A. fischeri don’t produce light, however. This behavior occurs only when the bacteria reach a specific population density within the squid’s light organ. Thus, bioluminescence is controlled by quorum sensing. The cells produce an autoinducer called homoserine lactone. Once enough of this inducer accumulates, the necessary genes for bioluminescence are transcribed.
Ever-resourceful scientists have taken advantage of the fact that all of the light-producing genes are contained within the luxCDABE operon. They moved the 9 kb operon into other organisms like E. coli, where it is often used to report on gene expression. Just engineer the fragment to sit downstream of your gene of interest - when that gene is transcribed, the downstream lux genes will be transcribed as well. Voila! Light!