Table 1.

General functions of neurotransmitters produced by the gut microbiota.

Gut Microbiota	Neurotransmitters Produced	Function	References
Bifidobacterium infantis, Candida spp., Streptococcus spp., Escherichia spp., Enterococcus spp.	Serotonin (5-HT; 5-hydroxytryptamine)	Involved in regulating behavioral and biological functions in the body, such as mood Plays a role in both psychological processes in the central nervous system (CNS) and peripheral tissues, such as the bone and gut	[24]
Bacillus spp., Lactobacillus spp.	Dopamine	Vital role in motor control, learning, memory formation, and the stress response Regulating carbohydrate and fat metabolism in the body Improvement in memory recovery	[25,26]
Escherichia spp., Bacillus spp., Saccharomyces spp.	Norepinephrine
	Noradrenaline
Lactobacillus spp., Bifidobacterium spp.	Gamma-aminobutyric acid (GABA)	Control of excitatory and inhibitory neurotransmission	[27]
Lactobacillus spp.	Acetylcholine	Primary excitatory neurotransmitter Influences synaptic plasticity and reinforces neuronal plasticity Cortical dynamics during learning and changes neuronal excitability Neurons are responsible for changing environmental conditions faster	[28,29]
Bifidobacterium spp.	Gamma-aminobutyric acid (GABA)	Reduces anxiety, fear, and stress	[30,31,32]
Bifidobacterium spp., Bacillus spp.	Tryptophan	Improves cognitive functions