I haven’t read the primary literature that describes this observation, but the finding that gut bacteria, through some mode of chemical signaling can modulate the neurochemistry of the organism has been an increasingly published area of research.
This study and others like it lend support to the idea of a microbiome-gut-brain axis. The hypothesis that a dynamic relationship exists between the microbiotic flora of our guts and the nervous system, mediated through the bundle of nerves that line our gut. But bidirectional communication between the brain and the gut isn’t really that novel of an idea, especially when it’s something we’ve probably all experienced by means of the butterflies in the stomach, or perhaps you’ve experienced the need to…evacuate the bowel when presented with an anxiety producing situation, like mounting a fight or flight response.
This study investigated a nuanced aspect of the relationship between gut flora and the serotonergic system, using mouse as the animal model.