Gut bacteria can produce neurotransmitters that interact with the brain via the vagal nerve to calm anxiety.

“The microbiota of the gut, if they're diverse and you have enough of them, are going to produce the short chain fatty acids that are critically required for the conversion of tryptophan, which again is going to come from your diet, into the serotonin of your gut, which in turn is going to be relayed. And it's not the actual serotonin that's relayed, but the presence of serotonin at sufficient levels in the gut, is communicated by the vagus nerve, up to the dorsal raphe nucleus.”