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Seeing viruses in a new light

Have you ever wanted to play the villain in a movie and create your own virus? It’s easy: combine one molecule of genomic nucleic acid, either DNA or RNA, and a handful of proteins, shake, and in a fraction of a second you’ll have a fully-formed virus.

While that may sound like a bad infomercial, in many cases making a virus really is that simple. Viruses such as influenza spread so effectively, and as a result can be so deadly to their hosts, because of their ability to spontaneously self-assemble in large numbers.

If researchers can understand how viruses assemble, they may be able to design drugs that prevent viruses from forming in the first place. Unfortunately, how exactly viruses self-assemble has long remained a mystery because it happens very quickly and at very small length-scales.
A system is needed to track nanometer-sized viruses at sub-millisecond time scales. And now, lead author Sanli Faez from the Leiden Institute of Physics has developed such a technique in collaboration with researchers at Harvard University, MIT, the Leibniz Institute of Photonic Technology, the University of Jena, and Heraeus Quarzglas, a manufacturer of fiber optics. The team, led by Vinothan Manoharan (Harvard University), published their paper in ACS Nano.

There are two main challenges to tracking virus assembly: speed and size. While fluorescent microscopy can detect single proteins, the fluorescent chemical compound that emits photons does so at a rate too slow to capture the assembly process. It’s like trying to observe the mechanics of a hummingbird’s flapping wing with stop-motion camera; it captures pieces of the process but the crucial frames are missing.

Very small particles, like capsid proteins, can be observed by how they scatter light. This technique, known as elastic scattering, emits an unlimited number of photons at a time, solving the problem of speed. However, the photons also interact with dust particles, reflected light, and imperfections in the optical path, all of which obscure the small particles being tracked.

To solve these problems, the team decided to leverage the outstanding quality of optical fibers, perfected over years of research in the telecommunication industry. They designed a new optical fiber with a nano-scale channel, smaller than the wavelength of light, running along the inside of its silica core. This channel is filled with a liquid containing nanoparticles, so that when light is guided through the fiber’s core, it scatters off the nanoparticles in the channel and is collected by a microscope above the fiber. The researchers observed the motion of viruses measuring 26 nanometers in diameter at a rate of thousands of measurements per second.

‘The main goal of our research has been to develop a general tracking platform based on elastic scattering to overcome the limitations of fluorescence microscopy,’ says Faez. ‘We worked with the virus as a notable demonstration of the advantages of using this platform. Apart from virus research, we can now use it for many different applications, for instance counting vesicles in the bodily fluids, which is too expensive for regular screening with existing methods based on electron microscopy. Cancer cells produce more vesicles than healthy cells, so if we can monitor someone’s vesicle count accurately enough, we can possibly develop a new diagnostic tool for cancer.’

Publ. 09-12-2015 10:41
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