FACTS ALREADY KNOWN o Serum concentrations of 5-HT are substantially reduced in mice reared in the absence of microbial colonization (germ-free GFD, compared to conventionally- colonized(specific pathogen-free [sPFD controls o Intestinal ECs are morphologically larger in gF versus SPF rats o Some species of bacteria grown in culture can produce 5 HTFACTS ALREADY KNOWN  Serum concentrations of 5-HT are substantially reduced in mice reared in the absence of microbial colonization (germ-free [GF]), compared to conventionally￾colonized (specific pathogen-free [SPF]) controls.  Intestinal ECs are morphologically larger in GF versus SPF rats.  Some species of bacteria grown in culture can produce 5- HT