Gut Bacteria Strike Back: Probiotic Reverses Alzheimer-Like Brain Damage in Rats

A single strain of gut bacteria may be able to undo some of the hallmark damage of Alzheimer’s disease in the brain, according to a new study from researchers at the ICAR-National Dairy Research Institute in Karnal, India. The team found that Limosilactobacillus fermentum NCDC701, a probiotic originally isolated from dairy sources, dramatically reduced amyloid plaque deposition, neuroinflammation, and oxidative stress in rats whose brains had been damaged by a combination of aluminium chloride and D-galactose, two well-established chemical triggers of Alzheimer-like pathology. The findings, published in Discover Toxicology, add weight to a rapidly growing body of evidence that the trillions of microbes living in our intestines can shape the fate of our neurons.

The experimental logic behind the study is rooted in what scientists call the microbiota-gut-brain axis, the bidirectional communication network linking the intestinal microbial community, the gut epithelial barrier, the immune system, and the central nervous system. In recent years, disruptions in this axis have been implicated in a range of neurological conditions, from depression and autism to Parkinson’s and Alzheimer’s disease. In Alzheimer’s specifically, an imbalanced gut microbiota can compromise the intestinal mucosal barrier, allowing bacterial products such as lipopolysaccharides and amyloid-like molecules to circulate, activate microglia, and drive chronic neuroinflammation, which in turn accelerates neurodegeneration.

To model the disease, the researchers injected Wistar albino rats intraperitoneally with aluminium chloride at 50 milligrams per kilogram of body weight and D-galactose at 100 milligrams per kilogram for seven consecutive days. Aluminium is a known neurotoxin that disrupts metal homeostasis and promotes iron-mediated lipid peroxidation, while D-galactose generates reactive oxygen species that damage proteins, lipids, and DNA. Together, the two chemicals reliably produce the pathological signature of Alzheimer’s in rodents: beta-amyloid plaque accumulation, neurofibrillary tangles, declining neurotransmitter levels, and widespread oxidative injury. The team then tested three different probiotic regimens, co-administration alongside the toxins, a prophylactic protocol in which the probiotic preceded the toxins, and a therapeutic protocol in which the probiotic followed the toxin exposure, and compared all of them against a memantine-treated positive control group.

The histopathological results were striking. Congo red staining of brain sections revealed that rats exposed to aluminium chloride and D-galactose developed dense amyloid plaque deposits, cerebral amyloid angiopathy, and neurofibrillary tangles in the hippocampus and cerebral cortex. In the rats that received L. fermentum NCDC701, whether before, during, or after toxin exposure, these abnormalities were markedly reduced, with hippocampal tissue appearing largely normal, cells intact, and no signs of necrosis. Notably, the probiotic-treated groups appeared to outperform memantine, the standard anti-Alzheimer drug used as a positive control, which left some residual plaques and angiopathy. In the colon, haematoxylin and eosin staining showed that the probiotic restored epithelial integrity and reduced the infiltration of inflammatory leukocytes and neutrophils that the toxin regimen had caused.

Biochemical assays using ELISA quantified the molecular shifts underlying these tissue-level changes. The toxin-exposed rats showed elevated concentrations of beta-amyloid peptides 1-40 and 1-42 in both brain and serum, along with depressed levels of the inhibitory neurotransmitter GABA and the monoamine serotonin. Supplementation with L. fermentum NCDC701 reversed both trends, lowering amyloid peptide concentrations and restoring GABA and serotonin toward control levels. This is consistent with the strain’s in vitro profile: the researchers measured GABA production by NCDC701 at approximately 99.43 millimolar, suggesting the bacterium is a substantial neurotransmitter factory. Because GABA decline is considered a contributor to Alzheimer’s symptomatology, and because reduced serotonin has been linked to the aggressive behaviors seen in dementia patients, the neuromodulatory capacity of this strain is of particular interest.

The probiotic also rebalanced the immune chemistry of both brain and colon. Pro-inflammatory cytokines IL-6 and TNF-alpha, which surged after toxin exposure, fell significantly in all three probiotic groups, while the anti-inflammatory cytokine IL-10 rose. At the transcriptional level, RT-qPCR showed that NCDC701 suppressed the expression of p65, a key component of the NF-kappaB inflammatory signaling complex, along with COX-2 and iNOS, two enzymes that amplify inflammatory and oxidative damage, in both brain and colonic tissue. Simultaneously, the strain boosted the activity of the antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase, which the toxin regimen had suppressed, thereby restoring the tissue’s capacity to neutralize free radicals.

One of the most clinically relevant findings concerned the gut barrier itself. The mRNA expression of tight junction proteins, zonula occludens-1, occludin, and claudin-1, dropped sharply in the colons of toxin-exposed rats, reflecting a leaky intestinal epithelium that permits bacterial endotoxins to enter circulation. L. fermentum NCDC701 restored the expression of all three proteins, effectively re-sealing the barrier. This matters because a compromised gut lining is thought to be a gateway event in the gut-brain axis model of neurodegeneration: it allows lipopolysaccharides and other microbial products to trigger systemic inflammation, activate microglia in the brain, and promote amyloid aggregation. By reinforcing the barrier, the probiotic may be cutting the disease process off at one of its upstream sources.

Deep sequencing of the V3-V4 region of the 16S rRNA gene in faecal samples revealed how profoundly the toxin regimen disturbed the gut ecosystem, and how effectively the probiotic repaired it. Aluminium chloride and D-galactose lowered the Shannon diversity index and the number of observed operational taxonomic units, signatures of dysbiosis. The Firmicutes to Bacteroidetes ratio, 1.435 in healthy controls, fell to 1.03 in the disease model but rebounded to approximately 1.4 in all probiotic-treated groups. Beneficial genera such as Lactobacillus, Prevotella, Ruminococcus, and Oscillospira, the latter known to support IL-10-producing regulatory T cells, all recovered after probiotic administration, while pathogenic taxa including Spirochaetes, Porphyromonadaceae, Coprococcus, Clostridium, and Allobaculum receded. The disease model also showed elevated Proteobacteria, a phylum increasingly recognized as a microbial marker of dysbiosis and inflammation, which declined with treatment.

Metabolite analysis tied these microbial shifts to functional chemistry. Gas-liquid chromatography of faecal samples showed that the disease model was enriched in propionate, echoing previous reports of elevated propionic acid in the blood of Alzheimer’s patients, while butyrate and total short-chain fatty acids were depressed. Probiotic treatment restored the short-chain fatty acid profile, increasing butyrate, a molecule that fuels colonocytes, strengthens epithelial tight junctions, and can modulate anti-inflammatory pathways through G-protein-coupled receptors. Because short-chain fatty acids can enter the circulation, cross the blood-brain barrier, and influence vagal, endocrine, and immune signaling, their restoration provides a plausible mechanistic bridge between the microbial changes observed in the colon and the biochemical recovery documented in the brain.

The authors are candid about the study’s limitations. The biochemical and molecular analyses used small groups of three animals per condition, a constraint imposed by resource limitations during the COVID-19 pandemic, and no human clinical trials have yet tested the strain. Larger and more diverse cohorts will be needed to confirm the statistical robustness and generalizability of the results, and translational studies will be essential before any commercial application. Nevertheless, the convergence of histological, biochemical, transcriptional, microbiological, and metabolomic evidence around a single probiotic strain is unusual and compelling. If future work validates these findings in humans, L. fermentum NCDC701 could represent a new class of adjunct therapy for Alzheimer’s disease, one that works not by attacking plaques directly in the brain, but by rebuilding the microbial and barrier defenses of the gut, and letting the microbiota-gut-brain axis carry the benefit upward.

Subject of Research: Probiotic modulation of the gut-microbiota-brain axis in an aluminium chloride and D-galactose-induced rat model of Alzheimer's disease

Article Title: Ameliorative action of probiotics on the neurotoxicological effect of Aluminium chloride and D-galactose

Article References: Dasriya, V. L., Kumari, M., Ranveer, S., Behare, P., Vij, S., & Puniya, A. K. (2025). Ameliorative action of probiotics on the neurotoxicological effect of Aluminium chloride and D-galactose. Discover Toxicology, 2(1), Article 6. https://doi.org/10.1007/s44339-025-00022-0

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00022-0

Keywords: probiotics, Limosilactobacillus fermentum, Alzheimer's disease, gut-brain axis, amyloid plaques, neuroinflammation, oxidative stress, GABA, short-chain fatty acids, gut microbiota, tight junction proteins, aluminium chloride