Table 3.

Functions of gut microbiota on Parkinson’s disease.

Functions	Bacterial species	Functional substance	Mechanism of action	Ref.
Neurotransmitters secretion	Lactobacillus spp., Bifidobacterium spp. (Y2) Streptococcus salivarius subsp. thermophilus	GABA	GABA secretion, regulate neural signaling in the enteric nervous system, control the growth of hormone secretion, control brain function and behavior	(Barrett et al., 2012; Pokusaeva et al., 2017)
	Escherichia spp., Saccharomyces spp. and Bacillus spp.	Noradrenaline	Noradrenaline secretion, regulate neural signaling in the enteric nervous system	(Shishov et al., 2009; Rogers et al., 2016)
	Streptococcus pp., Candida spp., Enterococcus spp. and Escherichia spp.	Serotonin	Serotonin secretion, regulate neural signaling in the enteric nervous system	(Özoğul, 2004; Shishov et al., 2009; Özoğul et al., 2012)
	Bacillus spp., E. coli, Hafnia alvei, Proteus vulgaris, Serratia marcescens	Dopamine	Convert l-tyrosine to L-DOPA, regulate neural signaling in the enteric nervous system	(Özoğul, 2004; Shishov et al., 2009; Rogers et al., 2016)
	Lactobacillus spp.	Acetylcholine	Acetylcholine secretion, induce epithelial cells to release molecules can regulate neural signaling in the enteric nervous system	(Reid, 2019; Rogers et al., 2016)
Fermentation of  dietary fiber	Prevotellaceae	Butyrate, acetate and propionate	Production of mucin and SCFAs, decreased SCFAs lead to increased intestinal permeability, exposure endotoxins, initiate excess α-syn expression and misfolding	(Sampson et al., 2016; Sarkar and Banerjee, 2019)
Rise serum lipopolysaccharide(LPS)	Enterobacteriaceae Gammaproteobacteria	LPS	Rise the serum LPS population, induce systemic inflammation, promotes α-synuclein deposition, increase LPS-α‐syn fibril formation	(Guo et al., 2013; Lin et al., 2019; Gorecki et al., 2019; Bhattacharyya et al., 2019)
Induce inflammatory responses	Ralstonia, Proteobacteria, Enteococcaceae	Pro-inflammatory cytokine	Increase of pro-inflammatory cytokine	(Keshavarzian et al., 2015)
Anti-inflammatory	Blautia, Coprococcus,and Roseburia and Faecalibacterium	Butyrate	The butyrate-producing bacteria such as Blautia, Coprococcus,Roseburia and Faecalibacterium decreased which have anti-inflammatory function	(Keshavarzian et al., 2015; Qiao et al., 2020)
Triggering factor in PD pathogenesis	Helicobacter pylori		Triggering factor in PD pathogenesis	(Keshavarzian et al., 2015; Çamcı and Oğuz, 2016)
Worsening of motivation	Bacteroides fragilis,Bifidobacterium		Low counts of Bacteroides fragilis related with worsening of motivation/activeness and Bifidobacterium decreasing related with hallucinations/delusions	(Minato et al., 2017)
Improve the bowel movement	Lactobacillus casei shirota		Improve the bowel movement, the number of fecal staphylococci was decreased	(Cassani et al., 2011)
Increase bile acid	Ruminococcaceae Lactobacillus	CA and DCA	Have a role in cognitive decline	(MahmoudianDehkordi et al., 2019; O’Donovan et al., 2020);
Converting levodopa to dopamine	Enterococcus, Lactobacillus Staphylococcus	Tyrosine decraboxylase (TDC)	TDC in genome of bacterias, have the ability of converting levodopa to dopamine	(Zoetendal et al., 2012; van Kessel et al., 2019)
Neuroprotective effects	B. animalis lactis, L. rhamnosus GG L. acidophilus	Butyrate	Induce BDNF and glial cell line-derived neurotrophic factor (GDNF) upregulated	(Srivastav et al., 2019)