THE ARISING · Longevity & Healthspan
Gut Bacteria From Sharp Old Rats Improved Memory in Forgetful Ones
Two people of the same age can differ sharply in how well their memory and thinking hold up. A new study in aged rats points to the gut as one reason: when researchers moved gut bacteria from one old rat to another, the recipient's cognition shifted toward that of the donor.
The work, published in the journal Immunity & Ageing on October 31, 2026, as an accepted article, is an animal study. A team from Shanxi Medical University and partner hospitals in China used aged male rats. They sorted the animals into two groups using behavioral tests, including the attentional set-shifting task, which measures mental flexibility, and the Morris water maze, which measures spatial memory. One group was cognitively unimpaired. The other was impaired.
The two groups carried different gut bacteria. According to the paper, sequencing of fecal samples showed that impaired rats had less diverse gut microbial communities, more Firmicutes and fewer Bacteroidetes. These are two major groups of gut bacteria.
The team then transferred pooled donor microbiota between groups through fecal microbiota transplantation after antibiotic preparation. The transfer experiments used seven recipients per group. Unimpaired rats that received bacteria from impaired rats developed cognitive deficits. Impaired rats that received bacteria from unimpaired rats showed improved flexibility and memory. The authors conclude that the gut microbiota played a causal role in shaping cognitive outcomes.
The study also traced a possible route from gut to brain. The researchers measured two blood markers, LBP and I-FABP, as indirect signs of how leaky the intestinal barrier is. Bacteria from unimpaired rats lowered both markers, which the authors say suggests better barrier function. Bacteria from impaired rats did not significantly change them.
Next came the immune system. In blood, impaired rats had fewer regulatory T cells, which help keep inflammation in check. They had more Th1 and Th17 cells, which drive inflammation. Their blood also held higher levels of pro-inflammatory cytokines, the chemical signals immune cells use to communicate.
These body-wide immune changes occurred alongside a pro-inflammatory shift in microglia, the brain's resident immune cells, in the hippocampus, a region central to memory. The authors propose an immune pathway connecting the gut and brain, but did not directly prove that these immune changes were necessary for the cognitive effects. Bacteria from unimpaired rats did the reverse: they restored regulatory T cell balance, raised anti-inflammatory cytokines and curbed the inflammatory shift in microglia, alongside improvements in cognitive tests.
For people, the finding is a clue about why brain aging varies so much. It supports further study of a possible pathway connecting gut bacteria, the intestinal barrier, the immune system and the brain. The authors describe the gut microbiota as a potential therapeutic target for age-related cognitive decline. The study used only male rats, so whether the same chain operates in people, or in females, remains open. This is not evidence that fecal transplantation or probiotic supplements treat human cognitive decline.
The team's funders include the National Natural Science Foundation of China, and the authors declare no competing interests. Watch for follow-up work testing whether the same gut-immune-brain chain appears in people and in female animals, and whether it can be shifted in ways that protect memory.
Source: Li and colleagues, Immunity & Ageing, October 31, 2026.