Norovirus grown in lab, with help from bacteria

After decades of effort, scientists have figured out the dirty little secret to growing a common stomach bug in the lab.

Bacteria found in feces help the diarrhea-causing, vomit-triggering norovirus infect human cells in plastic dishes, researchers report in the Nov. 7 Science.

“It’s a huge breakthrough for the field,” says immunologist Melissa Herbst-Kralovetz of the University of Arizona College of Medicine in Phoenix. Adding these bacteria along with the virus to human cells could finally give researchers a good way to test antiviral drugs and disinfectants, she says. 

Norovirus, a nasty microbe perhaps best known for wreaking intestinal havoc on cruise ship passengers, strikes about 20 million people in the United States per year. Healthy people typically conquer the infection within a few days, but the virus can be deadly for young children and older adults.

What’s more, good treatments for the illness don’t exist. Physicians simply try to keep patients hydrated, says study coauthor Stephanie Karst, a virologist at the University of Florida in Gainesville. Developing vaccines has been tricky too: One is in clinical trials now (SN: 11/16/13, p.16), but it’s based on a disabled version of the virus that can’t actually make copies of itself.

To test drugs that can fight the disease, scientists need to grow stocks of the virus in the lab. But until now, no one had been able to do it. “Really, the only source of norovirus is fecal material from infected people,” Karst says.

Researchers typically strain stool samples to get rid of bacteria and other fecal tidbits. Then, people collect the remaining “cleaned-up” virus for experiments. But Karst knew that bacteria can help some viruses infect intestinal cells. So her team wiped out mice’s gut microbes with antibiotics, and then hit the animals with a mouse version of norovirus.

In these mice, virus levels dropped. Norovirus seemed to need the bacteria to grow, the team concluded. To find out whether gut bacteria boosted norovirus infection in human cells too, Karst and colleagues dosed human immune cells with unfiltered stool — which is loaded with gut bacteria. Cells exposed to this “dirty” stool became tiny viral factories, cranking out more copies of norovirus than did cells hit with cleaned-up samples. 

Researchers had always cleaned stool samples“because we were trying to take out contaminants,” Herbst-Kralovetz says. “Now, lo and behold, it’s those contaminants that are promoting infection.”

Certain gut bacteria are speckled with sticky sugar molecules that grab onto viral particles and to human cells. The sugars help norovirus cling to the cells’ surface, Karst’s team found, possibly helping the virus enter and multiply.  

The new approach to growing human norovirus could help researchers find drugs to quash the bug, says study coauthor Jan Vinjé, a virologist at the U.S. Centers for Disease Control in Atlanta.

Until then, he says, people should stick to good old-fashioned hand washing. “It’s not so sexy,” he says. “But it’s the best way to prevent infection.”