"Do Gut Bugs Put the Brakes on Myelination?"... This mouse study found increased myelination in certain areas of the brain when gut microbiota were eradicated.
Presented at a poster session at the Society for Neuroscience (the world's largest organization of scientists and physicians devoted to understanding the brain and nervous system).
***This abstract was presented at a conference;conclusions should be considered preliminary until published in a peer reviewed journal.***
http://www.medpagetoday.com/MeetingCoverage/SFN/54276?xid=nl_mpt_DHE_2015-10-26&eun=g330925d0r

Microbe-free mice have hypermyelination of the prefrontal cortex