Dr Rachel McKendry, London Centre for Nanotechnology
Meera - This week Iím at the London Centre for Nanotechnology which is a joint venture of University College London and imperial College London. Iím here to see a new way of screening antibiotics which could help speed up the search for antibiotics against the ever-increasing hospital superbugs like MRSA. Iím here with Rachel McKendry, a reader in biomechanical nanoscience here at the London centre for nanotechnology and she helped developed this technique. Rachel, how have you been screening the effect of antibiotics against bacteria?
Rachel - Well, weíve been developing a novel nanomechanical approach to study antibiotics and to understand more about their modes of action and the mechanisms of superbug resistance.
Meera - And how have you looked into this?
Rachel - We use arrays of tiny silicon diving boards called cantilevers which we coat with different peptides found in bacterial cell walls. We then inject different antibiotics in solution and for our studies we focussed on vancomycin which is, in fact, one of the most powerful antibiotics that we have in the battle against resistant superbugs.
Meera - How does putting bacterial proteins on a diving board and then putting a solution of antibiotics help you learn about the effect of these antibiotics on the proteins?
Rachel - When the antibiotic binds to the peptide on the cantilever it causes the cantilever to bend by a very tiny amount, just a few nanometres. But we can detect this by shining a light on the very end of the cantilever and measuring its position on a photosensitive detector. What weíve found is the amount of bending is proportional to the concentration of antibiotic in solution. From this we can learn about the strength of the interaction, the binding constant which is a measure of how, essentially how powerful the drug is in the body.
Meera - Does this mean that the greater amount of bending you measure the greater the damage to the bacteria?
Rachel - Yes, exactly. Thatís our concept, that the biding generates huge mechanical consequences on the bacteria.
Meera - And just here in front of us weíve got an example of one of the silicon chips that you used to mount the bacterial proteins onto. Itís tiny but itís about half a centimetre long so we can actually see it.
Rachel - Youíre right in the sense. These are relatively large objects. They can be seen with the eye but if you look very closely at the top end you can see the silicon cantilever arrays. These are the diving boards at the end. Itís their thickness thatís the critical factor in determining their properties. Theyíre 500microns long. Thatís half a millimetre long, 100 microns wide Ė thatís typically the width of a human hair but the thickness is only 900nanometres. This means that it can detect very tiny changes in forces at the surface of the cantilever.
Meera - A key part of this experiment was that you used bacterial protein from bacteria that were resistant to antibiotics and also ones that were sensitive so you could see the difference between resistant and sensitive bacterial strains.
Rachel - Yes, the peptides differ by a single amino acid mutation and the mutation confers a deceptively simple change in the way that the drug works. It deletes a single hydrogen bond from the pocket between the antibiotic and the peptide and the binding of the antibiotic to the peptide found in drug-resistant bacteria is 1000-fold weaker than those found in drug-sensitive bacteria. This renders the drug therapeutically useless so weíve been fascinated with understanding this process and essentially hope in the future that we can design new antibiotics that combine to these peptides found in resistant superbugs.
Meera - One of the true benefits of this particular technique is that it can screen the effectiveness of antibiotics quite quickly. What makes this so much quicker than the other technique currently being used?
Rachel - Firstly that itís label-free. This means that you donít need to use fluorescent or radioactive probes as you might have to with other technologies and this has an advantage in terms of time and cost. But also labels can potentially perturb the way a biomolecule works. Other advantages are that they are immediately compatible with silicon microfabrication technology and what I mean by that is that itís possible to scale-up the number of cantilevers, readily for high density arrays to screen potentially thousands of drugs per hour. We hope that it will potentially provide a new way to understand how antibiotics work and hopefully develop more antibiotics in the future.