Table 1.

Gut bacteria associated with the synthesis of key neurotransmitters

Genus/species	Neurotransmitter (precursor)	CNS effect	Peripheral effect	Psychiatric conditions related to dysregulation	References
Candida; Streptococcus; Escherichia; Enterococcus; Lactobacillus bulgaricus	Serotonin (tryptophan)	Motor control, cerebellar regulation, synaptogenesis, addiction, emotion, memory, stress	Circadian rhythm, gut motility, body temperature, visceral pain, appetite, modulation of immune response	Depression, IBS, autism, Down's syndrome	Arreola et al., (2015), Gulesserian, Engidawork, Cairns, and Lubec (2000), Halford and Blundell, (2000), Hood et al. (2006), Jenkins et al. (2016), Leonard, (2010), Mazzoli and Pessione (2016), Marks et al. (2009), Meneses and Liy‐Salmeron (2012), Müller and Homberg (2015), Rogers et al. (2016), Stasi, Rosselli, Zignego, Laffi, and Milani (2014), Warren and Singh, (1996), Whitaker‐Azmitia (2001)
Corynebacterium glutamicum; Lactobacillus plantarum; Lactobacillus paracasei; Lactobacillus lactis; Brevibacterium lactofermentum; Brevibacterium flavum	l‐glutamate	Excitatory, brain development, synaptic plasticity		Generalized anxiety disorder, depression, bipolar, schizophrenia, neurodegeneration	Abdou et al. (2006), Boonstra et al. (2015), Cherlyn et al. (2010), Femenía, Gómez‐Galán, Lindskog, and Magara (2012), Hyland and Cryan (2010), Meldrum (2000), Yoto et al. (2012)
Lactobacillus; Bifidobacterium; Escherichia coli; Pseudomonas	GABA (l‐glutamate)	Inhibitory, anxiolytic	Myorelaxant, moderates intestinal motility, gastric emptying, gastric acid secretion, and inhibits GI carcinogens and tumor growth
Bacillus, Serratia, E. coli	Dopamine (l‐Dopa)	Reward‐motivated behavior, motor behavior, cognition, emotion	Stimulates exocrine secretion, inhibits gut motility, and modulates sodium absorption and mucosal blood flow	Schizophrenia, Parkinson's disease, depression, anxiety, addiction	Di Chiara and Bassareo (2007), Eisenhofer et al. (1997), Freestone (2013), Grace (2016), Lyte (2011), Meyer and Feldon (2009), Scheperjans et al. (2015), Shishov, Kirovskaya, Kudrin, and Oleskin (2009)
Bacillus; E. coli; Saccharomyces	Norepinephrine (dopamine)	Stress hormone, attentiveness, emotion, sleep, learning	Mediates growth and virulence of potentially pathogenic bacteria	Depression, schizophrenia	Freestone (2013), Moret and Briley (2011), Yamamoto and Hornykiewicz (2004)
(dependent on tryptophan production and serotonin synthesis)	Melatonin (serotonin)	Circadian rhythm, mood	Gastrointestinal function, protects against gut permeability, anti‐inflammatory, antioxidant, analgesic	IBS, multiple sclerosis, autism, Alzheimer's, mood disorders	Fornaro, Prestia, Colicchio, and Perugi (2010), Ghorbani, Salari, Shaygannejad, and Norouzi (2013), Ortiz, Benítez‐King, Rosales‐Corral, Pacheco‐Moisés, and Velázquez‐Brizuela (2008), Veatch, Goldman, Adkins, and Malow (2015), Wong, Yang, Song, Wong, and Ho (2015)