Serotonin is an important neurotransmitter in the brain and dysregulation of serotonin levels has been associated with various disorders, including depression and autism spectrum disorder (ASD) . Serotonin itself has been hypothesized to not cross the blood brain barrier . There is growing evidence that suggests the gut microbiome communicates with the brain in a bidirectional manner through the vagal nerve and through biomarkers that can cross the blood brain barrier (BBB). One of these biomarkers that can cross is the amino acid tryptophan (Trp), the precursor to serotonin. hydroxyindoleacetic acid (5-HIAA) levels, a major metabolite of serotonin, and increased circulating kynurenine and Trp . On the other hand, administration of Lactobacillus johnsonii probiotic in rats decreased kynurenine levels . These studies demonstrate that while Trp is not synthesized by mammalian cells, there are still biosynthetic and degradation pathways that alter Trp levels.