These viruses invade gut bacteria, but what they do is still a mystery. Scientists have identified more than 70,000 previously unknown viruses that live in the human gut and infect the bacteria that live there — but how they impact our bodies is a mystery. The gut microbiome, or the community of microbes that we carry around in our digestive system, plays an important role in food digestion and regulating the immune system, Live Science previously reported. But many studies have also linked imbalances in gut microbes to conditions including liver disease, obesity and allergies. Yet shockingly little is known about the microbiome. Although the microbiome includes a variety of microorganisms — including bacteria and viruses — previous studies have focused mainly on gut bacteria because they are easier to detect. In the new study, a group of researchers used a method called metagenomics to identify the viruses. This method involves analyzing all of the genetic material from a community of microbes together and then mapping the individual sequences found to specific species. They analyzed more than 28,000 gut microbiome samples taken from 28 countries. This process revealed complete genomes for more than 140,000 species of viruses living in the human gut. (A single person, however, carries around only a fraction of these species.) Though many types of viruses live in the gut, they focused on viruses that can infect bacteria, called "bacteriophages" or "phages" for short. The researchers limited their scope to bacteriophages because "we are still figuring out their role in human health," said lead author Luis Camarillo-Guerrero, a recent PhD graduate from the Wellcome Sanger Institute in the U.K. "It's probably safe to say that the vast majority of them are not harmful to us and are simply an integral component of our body microbiota." Phages may play a central role in the gut microbiome — for instance, by providing their bacterial hosts with advantageous traits and influencing how those bacteria evolve. "As bacterial communities are a critical component of our gut, it's not difficult to imagine that phages could be paying a key role in maintaining a healthy equilibrium in our intestine," Camarillo-Guerrero told Live Science in an email. However, there are known cases when phages have contributed to disease; for example, both diphtheria, a serious bacterial infection, and botulism, a serious illness that attacks the body's nerves, are caused by toxins that are encoded by phage genes. They published the genomes of these bacteria-invading viruses in a new database they created called the "Gut Phage Database," which can be used to guide further studies on these viruses, Camarillo-Guerrero said. "A genome is like the blueprint of an organism," he said. "The amount of information that we can extract from knowing only the DNA sequence of an organism is very large." The findings were published Feb. 18 in the journal Cell. Source
