Neurochemicals are highly conserved in evolution - bacteria, plants, insects, and fish all produce forms of the neurochemicals called the catecholemines. Thus it makes sense that bacteria in our gut can communicate directly with using, to some extent, the same “language” as our mammalian brains. Lactobacillus and Bifidobacterium species are known to produce GABA. Escherichia, Bacillus, and Saccharomyces produce norepinephrine. Candida, Streptococcus, Escherichia, and Enterococcus produce serotonin. Bacillus and Serratia produce dopamine, and Lactobacillus species produce acetylcholine. That’s pretty much the entire hit parade of major neurotransmitters