Liquid-Phase EM Allows Visualization of Viruses in Dynamic Systems

A team from Penn State has developed a new visualization technique that they used to see how human viruses move in high resolution in a near-native environment. Their live, 20-second-long movies of human viruses floating in liquid were recorded at near-atomic detail in an electron microscope. The same degree of information, immediately available as they record, may take up to 24 hours to acquire using traditional static imaging methods.  

“The challenge remained to view biological materials in dynamic systems that reflects their authentic performance in the body,” said Deborah Kelly, senior author on the paper published recently in Advanced Materials. “Our results show new structures and active insights of human viruses contained in minute volumes of liquid—the same size as respiratory droplets that spread SARS-CoV-2.”

Cryogenic electron microscopy (cryo-EM) is becoming the field’s gold standard for observing samples at or beyond atomic resolution, according to Kelly. The technique involves flash freezing the sample and focusing a beam of electrons through it. The electrons and the sample’s components interact, which is captured by detectors embedded in the instrument. Thousands of images can be processed to calculate what the item looks like in 3D—but more is needed to fully understand how the item functions in a more natural setting.

“While cryo-EM can tell us a lot of information, it still produces a static image,” said GM Jonaid, the paper’s first author. “With improved chips and a powerful direct detector on the microscope, we can accumulate a lot of movie frames to view how the sample acts in real time. We can see things how they exist—not just how we prepared them.”

The researchers used adeno-associated virus (AAV) as a model system to demonstrate their approach. They applied minute volumes of liquid solution containing AAV to the wells of specialized silicon nitride microchips. They then placed the microchip assemblies in the EM to examine the viruses in action.

“The images are very comparable to cryo-EM data, but the preparation was less complex, less technically involved,” Jonaid said. “Once we had the images, taken rapidly, like frames of a movie, we processed them just like we would any other high-resolution data.”

The results were videos of AAV moving in liquid, with subtle changes in the particle’s surface, suggesting that the particle’s physical properties change as it explores its environment, Kelly said. The resolution was close to three to four Angstroms.