Background: Intestinal microbiota and microbiota-derived metabolites play a key role in regulating the host physiology, including gut motility. We have previously shown that a wide variety of gut bacteria can produce 5-hydroxyindole from its precursor, dietary supplement and antidepressant, 5-hydroxytryptophan. When 5-hydroxyindole was administered orally in rats, it significantly accelerated the total gut transit time. Results: Here we show, using 16S rRNA sequencing, that 5-hydroxyindole has

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... o effect on the bacterial richness. No separation between the treated and untreated groups was detected based on their microbial composition as shown by principal component analysis. A significant increase in the abundance of only two genera, Shuttleworthia and Prevotellaceae_UCG-001, was detected in the 5-hydroxyindole treated group compared to the vehicle group. No significant association was found between the reduced total gut transit time as a result of 5-hydroxyindole administration and the cecal bacterial composition. Conclusions: Together, our study infers a marginal effect of 5-hydroxyindole or its subsequent accelerated gut motility on the cecal microbiota composition of rats. The results urge further investigation of the impact of 5-hydroxyindole oral administration in humans, which will support its potential application as a treatment for slow intestinal motility disorders.