As defined by John Cryan and Ted Dinan:

“In 2013, we defined a psychobiotic as a live organism that, when ingested in adequate amounts, produces a health benefit in patients suffering from psychiatric illness. As a class of probiotic, these bacteria are capable of producing and delivering neuroactive substances such as gamma-aminobutyric acid and serotonin, which act on the brain-gut axis. Preclinical evaluation in rodents suggests that certain psychobiotics possess antidepressant or anxiety-reducing activity. Effects may be mediated via the vagus nerve, spinal cord, or neuroendocrine systems.[1] Recently we have suggested broadening the psychobiotic concept to include prebiotics—the fiber that acts as food for the psychobiotics.[2]”

[1] Dinan, Timothy G., Catherine Stanton, and John F. Cryan. “Psychobiotics: A Novel Class of Psychotropic.” Biological Psychiatry 74, no. 10 (November 15, 2013): 720–26. doi:10.1016/j.biopsych.2013.05.001.

[2] Sarkar, Amar, Soili M. Lehto, Siobhán Harty, Timothy G. Dinan, John F. Cryan, and Philip W. J. Burnet. “Psychobiotics and the Manipulation of Bacteria–Gut–Brain Signals.” Trends in Neurosciences 39, no. 11 (November 1, 2016): 763–81. doi:10.1016/j.tins.2016.09.002.