The probiotic formulation (PF) consisting of Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 have been found to decrease stress-induced gastrointestinal discomfort. In this experiment, rats and human volunteers were tested. Rats were administered the PF formula for 2 weeks and tested in a conditioned defensive burying test, a screen model for anti-anxiety agents. In clinical trials, volunteers were randomly assigned to double-blind, placebo-controlled groups with PF administered for 30 days and assesed with a variety of tests: the Hopkins Symptoms Checklist, the Hospital Anxiety and Depression Scale, the Perceived Stress Scale, the the Coping Checklist and and 24 hour urinary free cortisol.

It was found that daily subchronic administration of PF significantly reduced anxiety-like behavior in rats and allieviated stress in volunteers. L. helveticus R0052 and B. longum R0175 taken together reduce anxiety-like behavior in rats and beneficial psychological effects in human volunteers.

Messaoudi, Michaël, et al. "Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects." British Journal of Nutrition 105.5 (2011): 755-764. 

Gut microbiota composition is associated with newborn functional brain connectivity and behavioral temperament  

Little is known about the gut microbiome, but it appars to play a role in human behavior and development. In this study, researchers look at how variability in the gut microbiome affects infant development of the brain. Infant gut microbiome diversity was measured using fecal samples and infant functional brain network connectivity was assessed using a resting state functional near infrared spectroscopy (rs-fNIR). The results show that micriobiome diversity is linked to individual variability in network connectivity and causes differences in their behavioral temperment. Data also corroborates that increases biodiversity of microbiota in the gut is associated with frontal-parietal connectivity, which has been linked to positive mental health.

Kelsey, Caroline M., et al. "Gut microbiota composition is associated with newborn functional brain connectivity and behavioral temperament." Brain, Behavior, and Immunity 91 (2021): 472-486.

Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks  

The enteric nervous system (ENS) plays a vital role in gastrointestinal physiologic functions such as motility, fluid secretion, and blood flow. The gut consists of countless types of microbacteria that modulate host production of several signaling serationin (5-HT) and other hormones and neurotransmitters. 90% of 5-HT comes from the intestines, and 5-HT activation in the ENS has been linked to adult neurogenesis and neuroproduction. Results found that germ-free mice with a microbiota from normal mice modified neuroanatomy of the ENS, which is associated with the effects of 5-HT in the adult intestine. Modification of the 5-HT receptors and depletion of 5-HT have identified a link between gut microbiota and maturation of the ENS through 5-HT activation it's respectivce receptor. In conclusion, it was found that gut microbiota modulate the ENS anatomy via seratonin (5-HT) receptors.

De Vadder, Filipe, et al. "Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks." Proceedings of the National Academy of Sciences 115.25 (2018): 6458-6463.

Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve

Is there a direct effect on neurotransmitter receptors in the CNS from bacteria in the gut microbiota? If so, how is this manipulation achieved? In this experiment, the lactic acid bacteria Lactobacillus rhamnosus was found to modify GABA mRNA in the brain, increasing it in cortical regions and reducing it in the hippocampus, amygdala, and locus coeruleus, compared to control-fed mice. 

In one of the experiments, the experimental mice fed L. rhamnosus (JB-1) swam longer than the control fed mice in a water tub with no possible escape. However, this effect dissapeared when the vagus nerve was severed in the experimental group. There was no effect on severing the vagus nerve in control mice. The neurochemical and behavioral effects were absent in vagotomized (removed vagus nerve) mice, showing the vagus nerve as a major communicator between the microbiota in the gut and the brain, forming the aptly named gut-brain axis via the vagus nerve.

Bravo, Javier A., et al. "Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve." Proceedings of the National Academy of Sciences 108.38 (2011): 16050-16055. 

Effects of microbiota transplantation and the role of the vagus nerve in gut–brain axis in animals subjected to chronic mild stress

Another paper on the gut microbiome affecting host behavior via the gut-brain axis vagus nerve. Fecal microbiota transplantation (FMT) has been casually relationship between stress-related microbiota and behavioral changes. The researchers' hypothesis was that FMT could present various physical and cognitive benefits in stressful situations and was regulated by the vagus nerve.

Mice were subjected chronic mild stress (CMS) protocol and seperated into variations of control, +FMT, and FMT+CMS. It was found that manipulation of the micriobiota could reverse the behavioral and biochemical changes from the CMS, and the vagus nerve influenced gut-brain axis response.

Ávila, Pricila Romão Marcondes, et al. "Effects of microbiota transplantation and the role of the vagus nerve in gut–brain axis in animals subjected to chronic mild stress." Journal of Affective Disorders 277 (2020): 410-416.