The oral microbiome is second only to the colon in the density of microbes living there. The mouth is really a collection of varied ecosystems that differ in composition. For instance, the microbiome of the teeth is different from that of the tongue that is, in turn, different from the microbiome of the gumline. The oral cavity is therefore teeming with hundreds, perhaps thousands, of species of bacteria, fungi, Archaea, and viruses. It is a virtual tropical jungle of life. But what is e...