Table 1.
Changes in microbiome composition, its metabolic status, and influence on host homeostasis and metabolome in AD and PD.
| Alzheimer’s Disease | ||||
|---|---|---|---|---|
| Microbiome Quantitative and Qualitative Changes | Changes in Microbiome Metabolome | Changes in Human Metabolome | Systemic Effects | Citation |
| Eubacterium deficiency; Escherichia, Shigella abundance | Increased LPS | Increased β-amyloid levels Increased plasma LPS levels |
Increased inflammatory state | [74,93] |
| H. pylori infection | Increased levels of H2O2 | Increased levels of homocysteine | Disrupted BBB integrity | [98] |
| Increased abundance of: Escherichia coli, Bacillus subtilis, Klebsiella pneumoniae, Mycobacterium and Salmonella species, Staphylococcus aureus, and Streptococcus spp. | Increased production of Aβ-amyloid plaques | Increased overall Aβ-amyloid plaques deposits | Disruption of proteostasis via molecular mimicry mechanism | [95] |
| Reduced: Clostridium sporogenes and Ruminococcus gnavus | Reduced levels of tryptamine | Reduced release of serotonin by enterochromaffin cells | Reduced levels of serotonin in gut | [48] |
| Not specified | Increased amount of secondary bile acids (deoxycholic acid, glycodeoxycholic acid, taurodeoxycholic acid or glycolithocholic acid) | Increased ratio between secondary bile acids and primary bile acids | Positive correlation between increased levels of secondary BAs, and hallmarks of AD | [91,100] |
| Parkinson’s disease | ||||
| Increased levels of Akkermansia muciniphila and Bilophila wadsworthia | Increased levels of S2-and SO3- Increased degradation of taurine-conjugated bile acids |
Disrupted transsulfuration pathways Reduced levels of glutathione |
Increased oxidative stress peripheral and neuroinflammation | [135,139] |
| Reduced levels of Prevotellaceae, Faecalibacterium, and Lachnospiraceae | Reduced SCFAs production, increased proteolytic activity (production of p-cresol or phenylacetylglutamine) | Increased SCFAs concentration in plasma Possibly connected to lower levels of methionine in serum |
Impaired gut-barrier function, increasing the risk of infection with enteric pathogens and boosting the α-synuclein formation | [119,122,123] |