Injection with a specific probiotic strain of bacteria (called Mycobacterium vaccae) prevents inflammation in the brain caused by stress and reduces stress-induced anxiety behavior in mice.
Another meta-analysis found that probiotics decrease anxiety values in people with anxiety (compared to controls) in 10 different studies. Several studies have identified the gut-brain relationship is bidirectional such that changes in the gut microbiome can affect behavior, and behavioral changes (ie. psychological stress) can affect the gut flora.
There are several proposed mechanisms including immune system modulation (ie. inflammation) and changes in brain neurotransmitters via vagal nerve stimulation. More studies still need to confirm the efficacy of probiotics for the treatment of anxiety but results look promising.