Engineers Develop More Than Tenfold Improvement In Measuring Virus Infectivity

The more sensitive detection system - developed by chemical and biological engineer John Yin and graduate student Ying Zhu - could help clinicians provide more individualized and finely tuned therapy to patients and aid drug developers in better characterizing viruses. Currently, there are no methods for measuring small amounts of resistant viruses. The system also could give scientists the beginnings of a new way to understand how viruses spread in people.

Plaque assays are regarded as the gold standard for measuring virus infectivity. The technique involves introducing virus particles into a Petri dish containing millions of healthy cells. Technicians cover the cells with a gel that keeps the virus from flowing freely. A visible pattern of dead cells forms as the virus moves through its host. Counting those dead spots gives the measure of infectivity.

"It's sort of like observing a pandemic in a Petri dish," says Yin. "What we've found is an opportunity to amplify the signal that a virus can give when it infects. The traditional idea is that one tries to prevent the flow of the virus particles in these types of cultures to avoid making a mess, but we've observed that flows can be quite directed and actually enhance the readouts for infection."

As described in the current issue of the Journal of Virological Methods, Yin's team replaced the layer of gel with a standard liquid growth medium. Where the gel restricted virus particles, causing neighboring cells to be more likely targets for infection, the liquid medium allows a virus particle to jump to cells well beyond its neighbors. The resulting path of destruction creates patterns like those of a comet or firework.

"Think of sitting on an airplane. If someone has a cold and the ventilation is not on, then those neighboring that person might get sick, but not the whole airplane. With the ventilation on, the cold can get to someone 20 rows away," says Yin. "The fluid in the Petri dish is like the ventilation on the airplane."

Observing virus infectivity in fluid rather than gel is not new, but the patterns reminded Yin of the way a coffee cup stain dries into a ring. Physicists had studied coffee stains and found that microflows carried the brown coffee particles into a ring formation as the stains dried. Microfluidics were also at play with the viruses multiplying in the liquid overlay. The plagues, or dead zones, flow toward the edges of the dish.

"No one has ever learned how these patterns were formed in the fluid medium," Yin says. "As engineers, we combined the visualization with a quantitative method to get at how drugs could effect these comet formations. We're using the physics of fluid flow to learn more about biology. For me, that is a really appealing way to try to learn something about biology. There is a lot going on in there. There are flows at work that people have never really appreciated."

Yin's work in funded in part by the National Science Foundation. His team is seeking patents on elements of their work through the Wisconsin Alumni Research Foundation.