Though viruses are the most abundant life form on Earth, our knowledge of the viral universe is limited to a tiny fraction of the viruses that likely exist. In a paper published this week in the online journal mBio, researchers from the University of Pittsburgh, Washington University in St. Louis, and the University of Barcelona found that raw sewage is home to thousands of novel, undiscovered viruses, some of which could relate to human health.

There are roughly 1.8 million species of organisms on our planet, and each one is host to untold numbers of unique viruses, but only about 3,000 have been identified to date.

To explore this diversity and to better characterize the unknown viruses, Professor James Pipas, Distinguished Professor of Biological Sciences Roger Hendrix, and Assistant Professor Michael Grabe, all of the Department of Biological Sciences in Pitt's Kenneth P. Dietrich School of Arts and Sciences, are developing new techniques to look for novel viruses in unique places around the world.

With coauthors David Wang and Guoyan Zhao of Washington University in St. Louis and Rosina Girones of the University of Barcelona, the team searched for the genetic signatures of viruses present in raw sewage from North America, Europe, and Africa.

In the paper, titled "Raw Sewage Harbors Diverse Viral Populations," the researchers report detecting signatures from 234 known viruses that represent 26 different families of viruses. This makes raw sewage home to the most diverse array of viruses yet found.

"What was surprising was that the vast majority of viruses we found were viruses that had not been detected or described before," says Hendrix.

The viruses that were already known included human pathogens like Human papillomavirus and norovirus, which causes diarrhea. Also present were several viruses belonging to those familiar denizens of sewers everywhere: rodents and cockroaches.

Bacteria are also present in sewage, so it was not surprising that the viruses that prey on bacteria dominated the known genetic signatures.

Finally, a large number of the known viruses found in raw sewage came from plants, probably owing to the fact that humans eat plants, and plant viruses outnumber other types of viruses in human stool.

This study was also the first attempt to look at all the viruses in the population. Other studies have focused on bacteria, or certain types of viruses. The researchers also developed new computational tools to analyze this data. This approach, called metagenomics, had been done before, but not with raw sewage.

The main application of this new technology, says Hendrix, will be to discover new viruses and to study gene exchange among viruses.

"The big question we're interested in is, 'Where do emerging viruses come from?'" he says. The team's hypothesis is that new viruses emerge, in large part, through gene exchange. But before research on gene exchange can begin in earnest, large numbers of viruses must be studied, the researchers say.

"First you have to see the forest before you can pick out a particular tree to work on," says Pipas. "If gene exchange is occurring among viruses, then we want to know where those genes are coming from, and if we only know about a small percentage of the viruses that exist, then we're missing most of the forest."