Table 2. Gut microbiota alterations in elderly controls and AD patients.

Bacterial species	Alterations of abundance		Mechanisms	References
	Elderly	AD patients
Clostridiales IV	↓	↓	Butyric acid is associated with resistance to inflammation and aging	Liu et al. [74]
Clostridiales X, α	↓	↓↓	Norepinephrine, acetylcholine and other neurotransmitters are related to cognitive and memory functions	Wall et al. [75]
Lactobacillus	↓	↓↓	Conversion of glutamate to GABA. Cognitive impairment may be due to disorders of the GABA system	Zhuang et al. [21]
Bifidobacterium	↓	↓↓	Bacteria can provide energy through SCFA, which can be used to promote the synthesis and secretion of neurotransmitters and hormones and to reduce the inflammatory response	Vogt et al. [22]
Staphylococcus aureus	↑	↑↑	Bacteria can secrete Aβ. Abnormal accumulation of Aβ activates diverse cellular receptors, leading to release of inflammatory factors which trigger or intensify the inflammatory response	Zhao et al. [62]
Escherichia coli	↑	↑↑	Bacterial metabolites exacerbate peripheral inflammation and can promote Aβ aggregation and cytotoxicity	Radli et al. [76]
Cyanobacteria	↑	↑↑	The neurotoxic amino acid BMAA causes protein misfolding and is a possible mechanism for β-amyloid deposition in AD patients	Banack et al. [77]
Gram-negative bacteria	↑	↑↑	LPS production stimulates the release of many inflammatory factors, promoting an inflammatory response	Itzhaki et al. [78]
Streptococcus	/	↓	Promotion of disease through immune mechanisms	Li et al. [79]
Bacteroides fragilis, Eubacterium spp.	/	↓	Reduced anti-inflammatory protection	Cattaneo et al. [30]

Abbreviations: AD: Alzheimer disease; GABA: gamma-aminobutyric acid; SCFA: short-chain fatty acid; Aβ: β amyloid; BMAA: β-methylamino-L-alanine.