Paul Orwin alerted me to a pleasant bit of serendepity—I had earlier written about squid and Vibrio symbiosis, and now the latest of issue of Science has a brand new article on the very same topic, complete with beautiful pictures of developing squid light organs. Here's the abstract, and you can see that there's more than pretty pictures to this story:

Tracheal cytotoxin (TCT), a fragment of the bacterial surface molecule peptidoglycan (PGN), is the factor responsible for the extensive tissue damage characteristic of whooping cough and gonorrhea infections. Here, we report that Vibrio fischeri also releases TCT, which acts in synergy with lipopolysaccharide (LPS) to trigger tissue development in its mutualistic symbiosis with the squid Euprymna scolopes. As components of PGN and LPS have commonly been linked with pathogenesis in animals, these findings demonstrate that host interpretation of these bacterial signal molecules is context dependent. Therefore, such differences in interpretation can lead to either inflammation and disease or to the establishment of a mutually beneficial animal-microbe association.

It's a nice story—something we think of as a nasty toxin is an important signalling factor in Euprymna development, responsible for sculpting its light organ. If cephalopods were the scientists here instead of us clumsy terrestrial chordates, no doubt it would be called "light organ morphogenetic factor" (or "t'chok mnakop fa'torsch", in the language of our brainy hypothetical squiddies), rather than the damning "toxin". Of course, they'd be wrong, too—names can be misleading, and molecules don't have an intrinsic biological function. What they do depends entirely on context.

Oh, but here, everyone wants pictures. What the Vibrio bacterium does in the squid is to be funneled by a ciliated organ into small sacs in the light organ, where it forms colonies that a) secrete substances that change the morphology of the organ, most obviously inducing cell death that eliminates the ciliated tissues, and b) eventually start producing luminescent patches on the squid's underside. In the photographs below, we can see the light organ with (sym) and without (non-sym) Vibrio bacteria, colorfully stained for cell death in A and B, and in C you can see how the ciliated protrusion completely disappears in the presence of the bacteria.

Bacterial components induce light organ morphogenesis. (A and B) Confocal micrographs of nonsymbiotic [non-sym, i.e., uninfected] and symbiotic (sym) organ epithelial fields stained with acridine orange (green) and Lysotracker (red). (A) Hemocytes (arrowheads) within the appendage sinuses (s). (B) Apoptotic cells, yellow foci (arrows). (C) SEMs of epithelial fields before (stage 0) and after (stage 4) regression. Scale bar, 50 µm.

Koropatnick TA, Engle JT, Apicella MA, Stabb EV, Goldman WE, McFall-Ngai MJ (2004) Microbial Factor-Mediated Development in a Host-Bacterial Mutualism. Science 306(5699):1186-1188.