Table 1.

A brief summary of gut microbial species that potentially affect the gut–brain axis through regulation of different neurotransmitters.

Gut Microorganisms	Role Associated with Neurotransmitter Regulation	References
Campylobacter jejuni	Affect glutamate synthesis and its metabolism	[15]
Brevibacterium lactofermentum, Bacillus subtilis, Corynebacterium glutamicum, and Brevibacterium avium	Involved in conversion of L-glutamate into D-glutamate through glutamate racemase enzyme, thus disturbs glutamate metabolism	[15]
Streptococcus vestibularis	Involved in synthesis and degradation of several types of neurotransmitters related to glutamate synthesis, GABA degradation, and isovaleric acid synthesis	[18]
Erysipelotrichales, Bacteroidales, and Clostridiales	Affect glutamate activity through metabotropic glutamate receptors (mGluRs)	[19]
Lactobacillus and Bifidobacterium species	Regulate GABA	[25]
Bacillus and Serratia	Control dopamine	[26]
Saccharomyces, Escherichia coli, and Bacillus	Regulate norepinephrine	[27]
Candida, Escherichia, Streptococcus, and Enterococcus species	Regulate serotonin production	[27]