8-OHdG, 8-hydroxydesoxyguanosine; AAMI, age-associated memory impairment; AD, Alzheimer's disease; BBB, blood­brain barrier; CFU, colony-forming units; Cognitive decline; ELISA, enzyme-linked immunosorbent assay; F/B, Firmicutes/Bacteroidetes; GFAP, glial fibrillary acidic protein; HE, hematoxylin and eosin; IHC, immunohistochemistry; IL-6, interleukin-6; Iba-1, ionized calcium binding adaptor molecule-1; LPS, lipopolysaccharide; MCI, mild cognitive impairment; Microbiota­gut­brain axis; NF-κB; NF-κB, nuclear factor-κB; NMDS, non-metric multidimensional scaling; OTU, operational taxonomic unit; PAMP, pathogen-associated molecular pattern; Probiotics; RIG-I; RIG-I, retinoic-acid-inducible gene-I; SAMP8 mice; SAMP8, senescence-accelerated mouse prone 8; SYN, synaptophysin; TEM, transmission electron microscopy; TLR4; TLR4, toll-like receptor 4; TNF-α, tumor necrosis factor-α; VE-cadherin, vascular endothelial-cadherin; ZO-1, zona occluden-1