Laser tweezers picking up bacteria

11 July 2010

Interview with

Dr Clare Bryant, Cambridge University

Ben -   A few weeks ago on the Naked Scientists, we heard how a highly focused laser beam can be used as if it were a pair of tweezers - it forms an "optical trap" that allows us to manipulate very, very tiny objects.  Now this technology has allowed researchers to try some very novel techniques, such as finding out how cells respond to pathogens on a one to one basis.  And we are joined by Dr Clare Bryant who's a Cambridge University researcher and she has been doing just that.  Thank you for joining us Clare. 

Clare -   Thanks, Ben for having me.

Ben -   Why is it important that we try and find out how cells respond to each other or how cells respond to pathogens on this one-to-one basis?

Clare -   Okay, so a lot of work looking at host pathogen interaction has focused on a sort of population view of how cells respond to pathogens, and some of the work we've been looking at has suggested that in fact, cells don't respond in a universal homogenous kind of way.  And actually, to explore exactly how a pathogen interacts with a cell, we need to do it as an individual cell-pathogen interaction, and this has revealed a number of very interesting things, so we're particularly interested in how macrophages (which are phagocytic cells which gobble up bugs) respond to pathogens such as Salmonella which causes food poisoning.  And we've found that in fact, despite the fact we assumed that all macrophages will become infected by Salmonella, in fact, they don't.  Most of them in fact aren't infected at all which is very bizarre, considering that a macrophage should be the cell that gobbles up all the bugs that are out there.

 Salmonella bacteria

Ben -   So, so far we've really just been looking at the average. We've been looking what happens with this whole population.  If I can just get into the laser because I had a go with one of these, it's called a holographic assembler a few weeks ago in Bristol and I was amazed of the fact that I was moving these tiny, tiny beads of glass that are a third the size of a red blood cell, but I also had to wear eye protection in order to be in the room.  So how do you adapt to laser so that it doesn't damage cells?

Clare -   Okay, so what we've done is we use a laser which is set to a wavelength of 1, 064 nanometres and that's actually within the infrared range of lasers, and this has been shown to be able to be used to manipulate cells and the cells remain viable.  They remain able to proliferate.  And therefore, we're reasonably confident that that's not actually going to damage the cell.  The other thing we do of course, is we enclose the cells and the bacterium within an environmental chamber with a gas , the humidity level and the temperature is controlled so that everything is set up for maximum cell viability and bacterial viability, but it's really used in the laser in the infrared spectrum which should reduce any damage to the cell and the bacterium.

Ben -   So you've already mentioned that we've looked at how macrophages, big eaters, cope with Salmonella and the fact that some of them don't do quite what we expect.  What sort of things can we actually hope to learn from understanding these one-to-one interactions?

Clare -   So, we're very interested in how the macrophages are actually able to take up the Salmonella and how the Salmonella is actually able to get into the cell.  So, we're going to be able to do things at two sort of levels. So first of all, actually just picking up a bacterium which is actually quite a challenge in its own right because they move very, very fast.  It's like a massive computer game, trying to catch these things.  They have tails which makes them spin and roll, and run.

Ben -   They're some of the fastest things in the world for their body length, are they?

Clare -   They are indeed.  Yes, they are indeed.  You have to be very, very fast with your tweezers.  Once you've caught them, you then are able to take them up to the macrophage.  You're able then to look not just at how the bug and the macrophage sort of interact with each other.  We can measure the physical processes that are involved so we can measure the time it takes for the bacterium to be taken up by the macrophage.  We can also try to explore the kind of forces that are involved which is an element of bacterial infection we haven't really considered in any way at all.  And then the other thing we're able to do is we're able to use cells that lack specific receptors that we believe are important for taking up the macrophage, taking up the bacterium rather, so then we'll be able to see, okay, how do these receptors contribute to the process of the Salmonella uptake, but also to the physical processes that may be involved. And the counter to that is we can take bacteria that lack the specific proteins that are important for uptake into the cell and see how that really affects not only the physical interaction with the cell, but the uptake process itself.  So we're going to be able to understand the whole process that's involved.  Further to that macrophages are existence sort of a number of different phenotypes, so we'll be able to take different types of macrophage and see which ones are actually important for taking up the cells, which ones are important for taking up the bacteria and allowing them to grow, and which ones don't take them up at all and try and explore what the physical difference is in their cells actually are and why that affects the ability of the Salmonella to get into the macrophage.

 A macrophage of a mouse forming two processes to phagocytize two smaller particles, possibly pathogens

Ben -   Can we use this trick to be even more specific as well?  I was mentioning manipulating these tiny glass beads.  Can we just coat them with say, one particular protein we're looking at and then find out exactly how a macrophage responds to just this protein without all of the other factors that complement it?

Clare -   Yes.  So some of the preliminary work we did was to look at a toxin which  is present on Salmonella called endotoxin and we're able to coat beads with endotoxin, and endotoxin is believed to be anti-phagocytic and what we were able to do was to compare beads specifically coated with endotoxin, (the beads were the same size as the bacteria) and then compare those to beads that had been coated with endotoxin, and an antibody which is important it's called an opsonisation process which enhances phagocytosis.  And what we could see if we compared uncoated beads with endotoxin coated beads and opsonised beads was that the endotoxin beads were much, much, much slower to be taken up into the macrophages whereas the opsonised beads were taken up really quickly, as were the beads that didn't have any coating at all.  So there's a whole plethora of experiments we should be able to do to allow us to precisely investigate those kind of processes.

Ben -   And what is the next step for you?  What's the goal you really want to chase at the moment?

Clare -   So the goal for us at the moment is to really explore why some macrophages take up Salmonella and some macrophages don't take up Salmonella, and then the key question for me as a biologist that I'm really interested in is to explore which of the receptors are important.  I'm particularly interested in receptors that recognise various pattern associated molecules that sit on the outside of the bugs, and they drive the innate immune response of a macrophage to the bugs.  So I'm really interested on how those receptors specifically affect the uptake of Salmonella into the cell, and whether or not there's a link then between the uptake of the Salmonella into the cell and the immune process then occurs downstream of that.

Ben -   Well thank you ever so much.  We could have to leave it there.  That's Dr. Clare Bryant from Cambridge University.  She is using laser tweezers to really look at something we've never looked at before, and that's the way that cells respond to pathogens on a one-to-one basis.

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