Forget retinal scans or DNA swabs. Scientists have discovered a potentially new way to sort one individual from another using the "deposits" they leave behind.

A new study, published online in Nature, reveals that each of us harbors in our intestines a collection of genetically unique viruses. Even identical twins, who share 100 percent of their DNA, carry different viral genes in their gut, researchers discovered. These viral genes can be mapped from stool samples -- a personal gut check that is unique to that individual and could eventually be used to give insights into our health.

In the new study, Jeffrey Gordon and colleagues at Washington University School of Medicine in St. Louis and San Diego State University analyzed the DNA of frozen stool samples from four pairs of healthy identical twins -- all women -- and their mothers. They sequenced the viral genes isolated from the 12 fecal samples at three different periods over a year, tracking any fluctuations in the viral communities over time. The researchers also sequenced the DNA of all the microbes in the stool samples, allowing them to compare the viral and microbial communities.

"The study highlights a few things that we did not know before," said Raymond Schuch, an assistant professor of immunology at Rockefeller University in New York who was not involved in the research. First, every individual in the study had a unique viral "fingerprint" in the gut that remained stable over the year, even in genetically identical twins. The viral genes of identical twins were about as different as those of unrelated individuals, while bacterial genes were much more similar in the biologically related women than in strangers.

"This is the first comprehensive analysis to explore the genetics of viruses that live in the gut and how these viruses may affect our intestinal ecology," Schuch said.

Although we mostly associate viruses with disease, the bugs in our bellies actually keep us healthy by regulating our digestion and protecting us from invading pathogens. In fact, these viruses may act as a barometer to gauge the health of the digestive system as it responds to daily challenges or recovers from an illness, the researchers suggest.

Most of these viruses, known as bacteriophages or phages, infect and live inside bacteria that also reside in the intestines. Phages can influence the activities of microbes by preying on and killing harmful bacteria. In fact, some scientists are engineering phages to combat multi-drug-resistant pathogens.

Phages can also develop a symbiotic relationship with bacteria, helping them survive and proliferate. Phages do this by transferring DNA from one bacterial host to another. This genetic mixing can improve the microbe's metabolism and its ability to eradicate pathogens.

However, the bulk of this information comes from studying phages in oceans. Researchers still have a limited understanding of the genetic diversity and the precise roles viruses play in the human gut.

"Another key finding from the study is that phages in the human gut appeared to favor a symbiotic relationship, not a predatory one," said Matthew Haynes, an author on the study.

This means that the phages living in our guts provide bacteria with new genetic material, allowing them to adapt over time and cope with changes that occur in the gut, Schuch noted.

To add to these preliminary findings, the researchers plan to study viral genes in the intestines of infant twins -- both identical and fraternal -- to determine how viruses first "set up shop" in the gut and how the foods infants eat influence their viral and bacterial fauna.