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The Telltale Tail

A symbiotic relationship between squid and bacteria provides an alternative explanation for bacterial sheathed flagella.

By | May 1, 2014

YOU’RE KILLING ME: During normal development of squid light organs, colonizing bacteria shed immunogenic proteins from their flagellar sheaths, triggering cell death. (Arrows point to apoptotic cells.)UNIVERSITY OF WISCONSIN, CAITLIN BRENNAN

EDITOR'S CHOICE IN MICROBIOLOGY

The paper
C. Brennan et al., “A model symbiosis reveals a role for sheathed-flagellum rotation in the release of immunogenic lipopolysaccharide,” eLife, doi:10.7554/eLife01579, 2014.

The background
The bioluminescent bacterium Vibrio fischeri, whose flagellum is encased in a membrane-derived sheath, colonizes newborn Hawaiian bobtail squid (Euprymna scolopes), contributing a light source that the squid use for camouflage. Researchers have assumed that the sheath prevented the host’s immune system from reacting to proteins in the flagellum. But the sheath itself sheds an immunostimulatory protein called lipopolysaccharide (LPS), a feature that has puzzled scientists.

The finding
To better understand the role of the sheath, Edward Ruby of the University of Wisconsin–Madison and colleagues colonized squid with mutant bacteria that had either no flagellum or one that wouldn’t spin. They found that flagellar rotation releases LPS and is a crucial trigger for normal development of the squid’s light-emitting organ. Bacteria with nonrotating flagella elicited as little response from the squid as bacteria without any flagellum at all. “Even though [the bacteria are] sitting right next to the [appropriate] tissue, they’re not able to cause the host to pull the trigger” on forming a proper light organ, says Ruby.

The partnership
The authors suspect that in the squid-vibrio symbiosis, the host immune response, rather than being defensive, may take part in alerting the squid to the presence of the beneficial bacteria.

The impact
The finding “reveals a previously unknown role of flagellar function and a previously unappreciated role of flagella-mediated LPS release,” says developmental biologist John Rawls of Duke University. Still, he notes, other questions remain, such as the sheath’s advantage for other bacteria and its broader biological function.

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