UW engineers develop more than tenfold improvement in measuring virus infectivity
Chemical and Biological Engineering Professor John Yin and graduate student Ying Zhu tweaked the standard system for measuring virus infectivity, digitized it, quantified it, analyzed it and discovered a method more than 10 times as sensitive.
Ultimately, a more sensitive detection system could help clinicians provide more individualized and finely tuned therapy to their patients, aid drug developers in better testing anti-viral drugs, and assist health scientists identify drug-resistant viruses. The system also could give scientists the beginnings of a new way to understand how some viruses spread in people.
Plaque tests are regarded as the gold standard for measuring virus infectivity. The technique involves introducing tens to hundreds of virus particles into a Petri dish containing millions of healthy cells. Technicians cover the cells with an agar or Jell-O-like substance that keeps the virus from flowing freely, yet allows it to infect nearby cells. As these infections spread they produce visible spots of dead cells called plaques. Counting the plaques gives a measure of infectivity. “It’s sort of like watching a pandemic spread in a Petri dish,” says Yin. “What we’ve found is a way to amplify the signal that a virus can give when it infects. The traditional idea is that one tries to prevent the random flow of the virus particles by using the agar, but we’ve found that fluid flows can be quite uniform 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 agar 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 flow to cells well beyond its neighbors. The resulting path of infection and 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 off, then the person's neighbors might get sick, but not people far away. With the ventilation on, the cold can spread 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 droplet on a table 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 plaques, or dead zones, flow toward the edges of the dish.
“It is still unknown exactly how these infection patterns form,” Yin says. “As engineers, we combined the visualization with a quantitative method to get at how drugs could affect these comet formations. We’re using the physics of fluid flow to amplify readouts from biology, which we then measure. For me, that is a really appealing way to try to learn something about biology. There are a lot of invisible flows going on in the simple petri dishes people use to culture cells. We are just beginning to appreciate these flows and make them useful.” Yin’s work is funded in part by the National Science Foundation. His team is seeking patents on elements of the work through the Wisconsin Alumni Research Foundation.