Table 2.

Roles played by different microorganisms residing in the gut.

No.	Organism	Positive ↑/Negative ↓ Effects	Subjects	Role	Reference
1.	Bacteroides fragilis	↑	AD patients	Protected against CNS demyelinating disease	[100,101]
			C57BL/6 mice	In pregnant mice showed an immediate significant diminished autistic behavior	[102,110,111]
2.	Lactobacillus casei	↑	SAMP8 mice	A decreased in anxiety symptoms	[112]
3.	Lactobacillus rhamnosus	↑	Wistar rats	Ameliorated the inflammation level in the brain	[103]
4.	Streptococcus thermophilus	↑	SJL/J mice	Robust effects on brain regions that control the central processing of emotions and sensation Degradation of Aβ 42 load	[113,114]
5.	Bifidobacterium infantis	↑	Sprague–Dawley dams rats	Normalized the immune response	[75]
6.	Campylobacter jejuni	↓	AD patients	Induced anxiety-like behavior Impaired memory	[104]
7.	Campylobacter rodentium	↓	C57BL/6 mice	Led to stress and contributed to behavioral abnormalities	[105]
8.	Porphyromonas gingivalis	↓	AD patients	Caused an inflammatory response in the liver, which subsequently led to neuroinflammation and causes neurodegenerative disease	[106]
9.	Eubacterium rectale	↓	AD patients	Leads to amyloidosis	[107]
10.	Lactobacillus acidophilus	↑	SAMP8 mice	Improved the impairment in neural proteolysis	[112,113]
11.	Lactobacillus johnsonii	↑	BB-DR rats Healthy humans	Improved gastric vagus nerve activity	[115,116]