Science Interviews


Sun, 21st Sep 2008

A Vaccine for MRSA

Gerald Pier, Harvard Medical School

Listen Now    Download as mp3 from the show Superbugs! Treating MRSA and C. diff

Gerry - The major work has been on infections caused by bacteria.  We focussed on two specific types: Staphylococcus aureus (of which one form is MRSA) and another organism called Pseudomonas aeruginosa which is a quite common organism.  Itís found in water but fortunately only in certain situation does it really cause a bad infection.

Chris - Why is Staph and MRSA such a problem?

MRSAGerry - Itís a little bit difficult to say why it has become a bigger problem recently.  Some of the issues seem to be greater exposure.  More people seem to be carrying MRSA.  Itís basically for years and years was pretty much only found in people in the hospital.  Now itís clearly out in people in the community.  Itís spreading more.

Chris - How many people have got it?

Gerry - About 30% of people are carry Staphylococcus aureus in their nose.  A smaller percentage Ė maybe around 2-5% carry MRSA.  Thatís more than it used to be.  Even though that sounds like a small number when you think of all the people out there itís really a large number of people overall.

Chris - What are you doing to try and stop that?

Gerry - One of the factors that has been very useful in making vaccines has been to extract the materials that bacteria put out on their outer surface.  That allows us to immunise individuals and get immune responses, usually antibodies, that can cause the bacteria to be killed.  Weíve discovered that there is a specific type of surface material thatís actually a sugar molecule.  Scientists call it a polysaccharide that if we chemically change it somewhat it causes a very good immune response.  Those antibodies are then capable of killing the Staph and preventing infections or perhaps limiting infections so that they donít become as severe.

Chris - What youíre saying is that people who donít have this vaccine Ė thereís something about the surface of the Staph that stops you making an immune response normally.

Gerry - Correct.  It turns out that one of the ways that bacteria avoid causing an immune response that would kill them is to trick your body into making the wrong kinds of antibodies.  The bacteria donít care you donít have those antibodies because they donít harm the bacteria.  Our job has been to try and figure out how to get the right kinds of antibodies that will kill the bacteria.

Chris - Do you know how they fought the immune system in that way?

Gerry - Itís a little bit complicated because it involves looking at this very detailed structure of this sugar molecule that coats the bacteria.  In general what we find is that the sugar molecule has projections from the sides and those projections are involved in causing the non-killing or bad antibodies to form.  Those antibodies bind to the bacteria ok but they donít cause other factors of the immune system to stick to the bacteria.  These other factors are essential for killing.  Itís the structure of this sugar molecule overall that gets the wrong kinds of antibodies mostly made.  What weíve done is weíve changed the structure so we get antibodies that then bring in these other factors that are needed to kill the bacteria.

Chris - So youíre just injecting people not with bacteria but with chemicals from the surface of the bacteria that drive this response?

Gerry - So far weíve only injected mice, rabbits and monkeys.  Although weíre fortunate that there is movement towards making this into a human vaccine, optimistically weíd hope perhaps within two or three years weíll see our first humans immunised with this material.  Itís going in the right direction but so far we only have data from animals.

Chris - Will the same chemical trick work on other infections because itís not just Staph that make us ill, is it?

Gerry - Weíre fairly optimistic that is the case.  Weíve shown with our laboratory studies that the same vaccine can be effective against E. coli which is also another common cause of infections in a hospital.  The bacteria that causes whooping cough also makes this.  The organism that causes plague makes it.  Thereís a lot of other organisms that have become quite problematic in the hospital setting because theyíre very resistant to the antibiotic drugs that we commonly use to treat infections.  In some cases we donít have any antibiotics at all.  Weíre hoping our vaccine or perhaps antibodies we make in the lab might be effective against these microbes as well.

Chris - Do you think thereís any risk that the bacteria could, once you start treating large numbers of people with this, could become resistant?  Bacteria change very rapidly.  Theyíve become resistant to lots of antibiotics.  Could they thwart your vaccine?

Gerry - This is always a possibility.  What one tries to do when you make a vaccine is to make it our of something that the organism has to have.  If it stops making it then it no longer causes infections.  In our laboratory studies weíve shown that if staphylococcus doesnít make this material itís much less able to cause an infection.  Weíre hoping thatíll be the case but weíre still a ways away from knowing that with any certainty.


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