The gut microbiome is hot like Hansel right now. It's all probiotic this, prebiotic that, and ferment all the things for the funky among us (me).
Dr. Will Bulsiewicz spent a while talking about microbiome on the very good Rich Roll podcast last summer and made a very interesting point:
Plants have a microbiome
Every single life on this planet either has a microbiome or is a part of the microbiome. You could make the argument that humans are part of a larger microbiome: planetary health.
Microbiome all the way down
(timestamped Overcast link)