Abstract
Alzheimer's disease (AD) is a progressive neurodegenerative disorder in which alterations in the microbiota-gut-brain axis has been increasingly investigated. However, the effects of individual cultivable bacterial isolates recovered from individuals with AD on host intestinal and neurobehavioral phenotypes remain insufficiently characterized. In this study, three bacterial isolates, Escherichia coli, Klebsiella pneumoniae, and Citrobacter pasteurii, were recovered from fecal samples collected from individuals diagnosed with AD and evaluated in healthy BALB/c mice following long-term oral administration. The effects of the isolates on physiological parameters, intestinal morphology and barrier-associated markers, inflammatory responses, gut microbial community composition, and neurobehavioral outcomes were assessed. The three isolates produced strain-specific effects, with C. pasteurii producing the most pronounced overall phenotype. Mice receiving C. pasteurii showed reduced body weight gain, increased spleen index, and shortened colon length, accompanied by elevated circulating inflammatory cytokines. Alterations in goblet cells, MUC2 expression, and tight-junction-associated proteins, including ZO-1 and claudin-1, were also observed, indicating alterations in intestinal barrier-associated structures. Gut microbiota analysis revealed strain-specific changes in microbial diversity and community composition, with the most pronounced alterations observed in the C. pasteurii group. In addition, C. pasteurii administration was associated with increased hippocampal inflammatory responses and astrocytic activation, whereas no apparent hippocampal neuronal loss was detected. Neurobehavioral testing revealed reduced locomotor activity and altered exploratory and behavioral responses, particularly in the C. pasteurii group. However, because reduced locomotor activity may influence performance in the novel object recognition and tail suspension tests, these findings should be interpreted cautiously. K. pneumoniae produced intermediate effects, whereas E. coli showed comparatively limited effects under the experimental conditions used. Collectively, these findings indicate that the tested bacterial isolates recovered from individuals with AD can produce strain-specific intestinal, inflammatory, microbial, and neurobehavioral alterations in healthy mice, with C. pasteurii producing the strongest overall phenotype. Importantly, the observed changes in gut microbial composition do not establish specific bacterial functions or pathogenicity, as microbial functional activity was not directly assessed. The present study does not establish that these bacteria are specifically associated with AD or that they cause AD progression; rather, it provides experimental evidence that individual cultivable bacterial isolates recovered from individuals with AD may differentially influence host phenotypes relevant to processes implicated in neurodegenerative disease. Further studies using larger human cohorts with appropriate healthy controls, validated AD models, and direct functional and mechanistic investigations are required to clarify the relevance of these observations to AD pathophysiology.