<p>Introduction In all the discussions about the community of microorganisms living in the human gut (the microbiome) and its "good" and "bad" bacteria; in all the science about how to manipulate bacterial populations to increase gut health; and in all the media urging greater awareness of the microbiome's influence on us all, there has been a missing link. The science of the microbiome has, until recently, largely neglected an active and potenti...