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Light Work
Symbiosis in a squid may help humans fight disease

By David Brooks
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Imagine if you could absorb bacteria from the environment that lets you glow at will. Wouldn't that be cool? Now imagine that the bacteria might also give you insight into diseases like cholera—that would be much, much cooler. It also explains why Cheryl Whistler has spent a decade investigating a wonderful creature called the bobtail squid and its bioluminescent bacteria.

"Everybody who sees my animals says they have charm and charisma and are beautiful," says Whistler, an assistant professor of molecular, cellular and biomedical sciences. "I love my squid, but what's important is that they are a unique model for studying basic interactions between microbes and their hosts."

Whistler is interested in the way the squid interacts with Vibrio fischeri, a bacterium that can be found floating in the open ocean and which has an interesting trait: it can glow. Even more interestingly, V. fisheri doesn't use this luminescent ability unless it is coddled in a special light organ of the bobtail squid, which somehow tells it to turn on at just the right times, and with just the right output, to camouflage the squid.

Waiting for food such as brine shrimp—better known to 8-year-olds everywhere as "seamonkeys"—to swim underneath, the bobtail squid hovers in the ocean at night near the surface. The squid's bioluminescent bacteria glow light blue, which the squid fine-tunes by adjusting the lobes of its light organ depending on how much moonlight or starlight there is. By balancing the luminescence of its V. fisheri with the celestial glow, the squid eliminates its shadow, allowing it to become invisible, at least from the point of view of its dinner.

This is a visually striking example of symbiosis, the interaction of two species for the benefit of both: the squid gets camouflage, the bacteria get a safe home with a good food supply. Virtually all species, including humans, depend on such bacterial symbiosis—"we have more microbes in our bodies than our own cells, by an order of magnitude," says Whistler—and yet this odd sort of teamwork is barely understood. The squid-Vibrio interaction provides a wonderfully simple model to study it, with exactly one species of bacteria associated with one light-emitting organ in one species of squid.

"Simple models are great for asking basic questions. The squid allows us to study how beneficial microbes are permitted to associate with the host, while the host can restrict bacteria it doesn't want," says Whistler. For example, it's unclear how the squid "knows" which of the millions of bacteria living in mucus around its light organ are V. fisheri, or how it communicates with them, although it seems that the genomic sequence GacA is involved. Understanding more about these issues could throw light on how we humans interact with our own beneficial bacteria, which is something of great interest to medicine.

But Whistler's charismatic cephalopods aren't valuable merely as a clean canvas on which to analyze symbiosis. V. fisheri is closely related to another member of the Vibrio genus that causes cholera, as well as one that causes gastroenteritis and one that infects wounds. Why the difference? "While V. fisheri became a good symbiont, these other Vibrios that are very closely related have gone down the dark side: Over time they've developed strategies to harm hosts to get what they want," says Whistler. "We think that by comparing the related pathogens and symbionts, we're going to get a unique perspective on virulence. If we could find something special in cholera, a specific virulence trait, perhaps we could target that to treat disease."

And that would be even cooler than being able to glow.

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