All microbes communicate with members of their own species and between species. This type of communication, called quorum sensing, allows bacteria to form rich social networks with other species as well as private conversations within their own species. Bacteria use quorum sensing to keep tabs on the density of members of their species, in relation to the density of other species, in order to perform fantastically synchronous events they could never accomplish working in isolation--like the successful invasion of a host. And while pathogen-host interactions are incredibly intricate, beautifully structured, and close to my heart (figuratively) I’ll be focusing on an incredible symbiotic relationship between a microbe and a squid: Vibrio fischeri and its life partner, the Hawaiian bobtail squid.

V. fischeri is a bioluminescent bacteria that fluoresces under high density conditions. If for example this bacteria is cultured in a dilute medium it won’t fluoresce at all. How exactly does V. fischeri know when and when not to fluoresce? It knows this because each bacterial cell releases highly specialized molecules (gram positive bacteria use small peptides) called autoinducers at diffuse in and out of each cell. Since each bacteria is constantly secreting these molecules, the more bacteria present in solution the higher the concentration of the autoinducer. Once a minimum threshold for the autoinducer is detected by V. fischeri, every single species member changes their gene expression in unison producing bioluminescence. Quorum sensing mechanisms are also highly conserved--in fact, they could be leveraged as a new wave of antibiotics or as a way to augment microbial conversations in order to improve the tasks bacteria already perform for us daily. For a more detailed overview, check out Steven T. Rutherford’s recent review on quorum sensing .

That’s all well and good but where does the squid fall into this story?

The Hawaiian bobtail squid (~5 inches long) lives in shallow waters off the coast of Hawaii. On the ventral side of the squid is a light organ which faces the ocean floor when it swims. This organ houses ~1010 to 1011 V. fischeri cells--that's a huge amount of bacteria--and this squid maintains it daily. Since the bobtail squid is nocturnal and lives in a few feet of water, moonlight and starlight penetrate this depth easily. This squid has evolved a camouflage, however. It has photoreceptors on its back that detect the amount of starlight and moonlight coming in, and using its ink sac as a shutter over the light organ, the squid can match the exact amount of moonlight and starlight coming into the water with the luminescence from its light organ, thus producing no shadow as it swims. This is thought to be an anti-predation strategy but I think of it more as one of nature’s klingon cloaking devices.

The squid also needs to do something about its dying culture in the light organ so when the sun rises, and the squid burrows itself into the sand, (s)he dumps out more than 90% of its culture into the ocean. And sure enough, now that the few remaining V. fischeri cells are left, they’re not producing enough of the autoinducer to meet the minimum threshold and therefore do not produce luminescence from the light organ. This beautifully synchronous event is intertwined with the squids circadian rhythm, producing light exactly when the squid needs it.

This relationship is so highly evolved that the squid can actually select between different V. fischeri strains for the flashiest of them all.