Most folks would typically consider bacteria to be either good or bad.  The bad such as those that cause infectious disease or the good like the normal flora that aid in digestion, but none would think of viruses having a dual nature as well. A group of scientists led by Jeremy Barr discovered that mucus, a viscid gel-like secretion rich in mucins that act as a protective lubricant from infectious agents, is more than just a barrier. Surprisingly, the active layer of mucus consists of bacteriophages that attack and kill infectious bacteria; such that, Barr later called them “friendly viruses”.  The group also found out that there are more phages in mucus than in mucus-free areas. For example, human saliva harbors about five phages for every bacterium, but mucus directly on the gums host nearly eight times more phages.

The group conducted experiments to confirm their new findings and hypothesis. They cultivated normal human lung cells that produce mucus and knockout lung cells that lost the ability to secrete mucus. The two groups are then incubated overnight with Escherichia coli. By the next morning about half the cells in each culture died, indicating that the presence of mucus made no significant difference. However, in their second experiment the group added a virus that targets E. coli to the mucus containing culture and saw that mortality rates significantly decreased which supported their hypothesis.

Although Barr and his colleagues showed that mucus filled with phage can kill bacteria, , it is still unclear whether or not they attack beneficial bacteria. They speculate that it is plausible that the host selects specific viruses to combat specific infectious agents. So the next steps for Barr and his group was to determine and study the purpose of this mutualistic relationship between human mucus and viruses.