Table 1.

Involvement of the gut microbiome in neurodegenerative diseases and their GM related plausible mechanisms.

S.no.	Microbiota dysbiosis	Possible mechanism/Pathology	Reference
Alzheimer’s disease (Human studies)
1.	Increased: H. pylori	Patients infected with H. pylori with AD showed high cognition impairment. Infection-induced cytokine levels are associated with increased gastric atrophy and linked cognitive impairment, causing tissue damage caused by the increased release of toxins.	Roubaud-Baudron et al. (2012)
2.	Increased: Escherichia/Shigella Decreased: Eubacterium rectale; Bacteroides fragilis	Increased Escherichia/Shigella and decreased anti-inflammatory Eubacterium rectale and Bacteroides fragilis are associated with increased pro-inflammatory cytokines, which may be correlated with a peripheral inflammatory state in cognitively impaired AD patients. Bacteroides fragilis is associated with increased production of lipopolysaccharide.	Cattaneo et al. (2017)
3.	Increased: K. pneumoniae, B. fragilis, and E. lenta Decreased: B. hungatei and B. proteoclasticus	Increased production of pro-inflammatory bacteria and decreased synthesis of butyrate establish potential nexus between AD and intestinal bacteria and modulate intestinal homeostasis with disbalance in pro-inflammatory bacteria and decreased butyrate. Butyrate presents in the colon that maintains the colonic epithelial barrier. In contrast, pathobionts like K. pneumoniae could accumulate extracellular amyloids, B. fragilis is associated with increased lipopolysaccharide production, and E. lenta is associated with severe gastric pathology.	Haran et al. (2019)
4.	Increased: Oscillospirales Decreased: Lactobacillales	Alteration in the gut microbiota in AD patients is one of the interesting fields. Interaction mechanisms between human microbiota and the brain might reveal interesting findings and uncover novel pathogenesis mechanisms. Lactobacillales synthesize antimicrobial peptides known as bacteriocins, which further prevent the growth of pathogens.	Askarova et al. (2021)
Alzheimer’s disease (Animal studies)
5.	Increased: Bacteroides fragilis	Bacteroides fragilis is associated with increased production of LPS, which BF-induced LPS cross-gut epithelium and enters the bloodstream, inducing a systemic inflammatory reaction and increase of pro-inflammatory cytokines. Increased Bacteroides fragilis could be used as a prognostic factor of AD in early life.	Borsom et al. (2023)
6.	Increased: Proteobacteria Erysipelotrichaceae	Profile of inflammatory-related bacterial Proteobacteria and Erysipelotrichaceae might contribute to disease progression.	Bäuerl et al. (2018)
Parkinson’s disease
7.	Increased: Verrucomicrobia, Mucispirillum, Porphyromonas, Lactobacillus, and Parabacteroides Decreased: Prevotella	Increased Bacteroides were found in PD patients with non-tremor than with tremor. Hence, Bacteroides abundance is associated with disease severity or worsened motor symptoms. Also, increased Bacteroides correlated with increased TNF-α and IFN-γ. Bacteroides induce inflammation by eliciting pro-inflammatory cytokines and disrupting the gut barrier.	Lin et al. (2019)
8.	Decreased: Lactococcus spp.	Decreased population of Lactococcus spp. was associated with an increase in lytic phages. A decrease in lactic acid bacteria may disrupt the intestinal barrier.	Tetz et al. (2018)
9.	Increased: Ralstonia Decreased: Blautia, Roseburia, and Fecalibacterium	increased in Ralstonia associated with pro-inflammatory bacteria. A decrease in the production of butyrate (SCFA) [Blautia, Roseburia, Fecalibacterium] is contributed to gut hyperpermeability.	Keshavarzian et al. (2015)
10.	Increased: Enterobacteriaceae Decreased: Prevotellaceae	Increased Enterobacteriaceae is associated with postural instability and gait difficulties. Prevotella synthesis of thiamine and folate. Decreased Prevotella might cause prevention in the synthesis of thiamine and folate.	Scheperjans et al. (2015)
Parkinson’s disease (Animal studies)
11.	Increased: Proteobacteria Decreased: Lachnospiraceae and Ruminococceae	Distribution in the gut microbiota is a hallmark of the motor and GI dysfunction in the mouse model of PD. Disbalance in microbiota causes microbial molecules to instigate gut-brain signaling that may impact neuroinflammation and causes PD pathologies like α-Syn aggregation.	Sampson et al. (2016)
Amyotrophic lateral sclerosis (Human studies)
12.	Increased: Bacteroidetes (Phylum) Decreased: Firmicutes	Increased Bacteroidetes and decreased Firmicutes reflected undermined condition in ALS condition. Bacteroides induce inflammation by eliciting pro-inflammatory cytokines and disrupting the gut barrier.	Zeng et al. (2020)
13.	Decreased: Prevotella spp.	Prevotella spp. Deficiency and modification in butyrate metabolism may have translational value for ALS treatment—prevotella synthesis of thiamine and folate. Decreased Prevotella might cause prevention in the synthesis of thiamine and folate.	Hertzberg et al. (2022)
14.	Decreased: Eubacterium rectale and Roseburia intestinalis	Several butyrate-synthesizing bacteria, which are crucial for gut integrity and inflammation regulation, were reduced in ALS patients compared to healthy controls.	Nicholson et al. (2021)
15.	Decreased: Firmicutes/Bacteroidetes ratio	Increased production of pathobionts and reduction in the synthesis of probiotic organisms in the intestine of ALS patients might down-regulated or upregulated production of NO, GABA, LPS, and SCFAs production, which might lead to increased pathogenesis of ALS.	Fang et al. (2016)
Amyotrophic lateral sclerosis (Animal studies)
16.	Decreased: Butyrivibrio Fibrisolvens, Escherichia coli, and Fermicus	A decrease in the level of antimicrobial peptides defensin 5-α was found in the ALS intestine, which was correlated with a decrease in the abundance of Butyrivibrio Fibrisolvens; this bacterium has a role in the regulation of proteins that regulate the intestinal epithelium permeability.	Wu et al. (2015)
Huntington’s disease (Human studies)
17.	Increased: Intestinimonas (genus) Decreased: Bilophila	In systemic inflammatory reactions in HD patients, Bilophila may have an anti-inflammatory effect. Pro-inflammatory IL-6 levels had a negative correlation with Bilophila. Intestinimonas and plasma levels of IL-4, an anti-inflammatory cytokine produced mainly by T-helper-2 cells, are positively correlated.	Du et al. (2020)
18.	Increased: Bacteroidetes Decreased: Firmicutes	Gut dysbiosis was associated with body weight impairment and motor deficit.	Wasser et al. (2020)
Huntington’s disease (Animal studies)
19.	Increased: Bacteroidetes Decreased: Firmicutes	Disbalance in the microbes in the gut/and gut microbiome leads to increase intestinal permeability and causes dysbiosis, which may cause HD pathology.	Stan et al. (2020)