Catching up (finally) on an article I've wanted to read for a while: "Slime City" in the July-Aug 2009 issue of Discover. (The magazine subscription is not mine, so I haven't had it on the list for over a year, just for a year or so.) Bacteria have a "quorum sensing" process that turns on some of their special abilities (whether toxic to the human body or beneficial to a squid, as below) ALL AT ONCE when they realize that there are enough of them around to make impact. How many people does it take to change a light bulb? Or, how many does it take to create light? Excerpts from the article, below:
[Geneticist Michael] Silverman talked about how bacteria make light inside the inch-long luminescent squid that live in the shallow waters off the Hawaiian coast. . . . Infant squid cannot glow until they excrete a mucuslike net to entrap the ubiquitous luminescent bacteria floating in the water. The squid draw captured bacteria into their "light pouches," where the bacteria are bathed in nutrients -- a diet richer than what they can find outside in the sea. In return, the bacteria . . . produce a dim blue-green light that is directed downward through small reflective organs in the squid to shine on the water below. When the squid swim at the ocean surface at night, hunting for shrimp, they are invisible to predators below because they look like moonlight on the water. . . .
Autoinducers (chemical signaling molecules . . .) control the switch that turns the light genes off and on. Each bacterium secretes a bit of this light-evoking substance into the environment. When a crowd of bacteria and their autoinducers become dense enough, the lights in all the bacteria switch on at once. "This counting of heads is called quorum sensing" . . . .