Caitlin Brennan, Harvard
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There is a unique partnership between a small 3-cm long squid called Euprymna scolopes and a bacterium called Vibrio fischeri. In return for sugars and amino acids, the bacteria inhabits specialised organs on the squid where they pump out light, providing the animal with camouflage. But how do these microbes talk with their hosts? Caitlin Brennan has found that molecules from the sheaths around the flagella that they use to propel themselves along are the critical component. She spoke to Chris Smith...
Caitlin - We’re really interested in how bacteria are moving around and using a flagellum to swim through different media and different liquids, and how this behaviour was important for interaction with host. To do this, we were using the squid-Vibrio mutualism, which is a model of host-microbe interactions and is used to look at issues of host specificity.
Chris - So, what did the squid do with the Vibrio bacteria?
Caitlin - The objective of the mutualism is to provide light for the squid, which is for counter-illumination which is a camouflaging mechanism and Vibrio fischeri is the only bacterium that's able to enter the specialised organ, grow there and provide the light that the squid needs for this behaviour.
Chris - So, how did you use that system to understand what the flagella does normally in relation to its host?
Caitlin - So because of the specificity between one bacterium and one organism, we’re able to probe various specific questions in terms of the conversations between these two organisms. In a way, we’re not able to in a more complex environment like the many, many microbes that are found in the human gut. That conversation is very, very complex because there are so many players.
Chris - So, do you think then there's some kind of signal coming out of the bacterium which is altering the behaviour of the squid and in order to achieve this mutualistic association?
Caitlin - So, we’ve shown in the past that there are specific signals put out by the bacteria that are really actually common signals to many bacteria. So, these include things like LPS or lipopolysaccharide and parts of peptidoglycan. These are surface molecules that are found on many, many bacteria, not just Vibrio fischeri. But what's unique about these signals is where they're expressed by Vibrio fischeri and where this squid is being exposed to them. They're being exposed to these signal molecules at a place only Vibrio fischeri is able to access. Exploiting this relationship to sense when Vibrio fischeri is there.
Chris - When you study this interaction, what is this telling you about that chemical conversation?
Caitlin - So, it’s telling us a lot about whether these molecules are the same molecules that are often causing really reactive host responses and a pathogenic relationship. If this was happening in your gut by the wrong bacteria, you'd be very sick and you'd have an inflammation. But here, it’s part of the natural progression of the interaction between Vibrio fischeri and the squid.
Chris - So, how did you actually study this? What did you do in order to unpick this?
Caitlin - So, we used three different strains – one bacterium that produces a flagellum and is able to swim, another that produces a flagellum, but can't actually rotate this flagellum so it can't swim, and one that doesn’t produce a flagellum at all. Using these three different strains, we’re able to pull apart what we’d actually do to the production of a flagellum and what we’d actually do to being able to swim to rotate that flagellum. What we found was that it didn’t matter whether Vibrio fischeri made a flagellum. If it couldn’t swim and if it couldn’t turn this flagellum then it was unable to cause this host respond. So, we looked a little more closely at the flagellum of Vibrio fischeri and it’s known that it’s covered in a sheath. This particular flagellar sheath is a continuation of the rest of the outer membrane of the bacterium. And so, what this is largely made of is lipopolysaccharide. And so, we wondered whether being unable to turn this prevented release of this molecule in a way that stimulated the host response by wild type Vibrio fischeri.
Chris - What is the effective bottom line here then? So, the bugs use their flagella to release or dispense LPS and that the host is responding to that LPS and in the case of the squid, for beneficial outcome.
Caitlin - This flagellar sheath is something that's been very poorly understood. I found a review at one point during my graduate work that I believe was from 1981. It was the only review there's ever been about a flagella sheath. And every single question they posed remains unanswered to this day. People have been assuming that the function of this sheath is to really prevent an immune response to the flagellum subunit. We just haven't had the tools to probe that in any bacteria that has a flagella sheath. This includes several important human pathogens like Helicobacter pylori and Vibrio cholerae. What we were able to do was use the squid-Vibrio symbiosis to highlight something that's potentially common to all these bacteria with sheath flagella and maybe important in how those bacteria are interacting with their respective hosts as well.