New Study Identifies Gut Bacteria-Produced Molecule That May Drive Alzheimer’s Progression
For decades, researchers have observed a consistent pattern: the gut microbial community of people living with Alzheimer’s disease looks distinctly different from that of people without the condition. Yet a core question has remained unanswered: are these microbial shifts a result of Alzheimer’s, a driving factor that contributes to disease development, or merely a neutral byproduct that occurs alongside illness? A growing body of research suggests the answer may not lie in the gut bacteria themselves, but rather in the small molecules they secrete that circulate throughout the human body. Now, a team of scientists says they have pinpointed one such molecule that could connect gut dysregulation to Alzheimer’s.
In a new study published in Nature Communications, the team identified imidazole propionate (ImP) — a metabolite generated by specific strains of gut bacteria — as a likely missing link between the gut microbiome and Alzheimer’s disease.
From the Gut to the Brain
The team’s proposed mechanism starts right in the gut, where certain strains of bacteria process the amino acid histidine through their normal metabolic pathways, producing ImP as a byproduct. A portion of this small molecule is absorbed into the bloodstream, where it travels throughout the body — and that’s where trouble begins, starting at the protective border surrounding the brain.
Study experiments show ImP erodes the function of the blood-brain barrier, the tightly regulated boundary that shields the brain from harmful substances in the blood and controls what compounds can cross into brain tissue. A weakened barrier lets more ImP slip into the brain itself, where the metabolite can interact directly with nerve cells. From there, ImP interacts with two of the most well-documented pathological markers of Alzheimer’s: amyloid beta and tau proteins.
In brains affected by Alzheimer’s, amyloid beta clumps together to form sticky plaques between neurons. Tau protein, meanwhile, undergoes abnormal structural changes that disrupt its normal function and trigger the buildup of toxic tangles inside nerve cells. Both processes drive progressive neuronal damage and are the two defining biological features of the disease. According to the study’s findings, ImP worsens both of these harmful processes. The researchers found ImP encourages the accumulation of amyloid beta plaques, and boosts abnormal modification of tau through a process called phosphorylation.
The full proposed chain of events looks like this: certain gut bacteria produce ImP → the metabolite enters the bloodstream → it weakens the blood-brain barrier to cross into the brain → once inside, it drives pathological changes to amyloid beta and tau that are central to Alzheimer’s. Of course, a logically consistent biological mechanism does not guarantee this process actually unfolds in the living human body. To confirm their hypothesis, the researchers first looked for evidence of the link in human populations, then tested individual steps of the mechanism in mouse models and lab-grown cells.
Evidence From Human Participants and Mouse Models
The research team analyzed ImP levels in blood samples from 1,196 cognitively healthy adults with an average age of 61.2 years. On average, participants with higher circulating ImP scored lower on standard cognitive function tests. They also had higher blood levels of two key Alzheimer’s biomarkers: pTau-217, a modified form of tau that is an early indicator of Alzheimer’s-related brain changes, and NfL, a protein that is released in larger quantities when neurons are damaged. Both biomarkers can be detected in blood and signal early brain changes long before noticeable Alzheimer’s symptoms appear.
When tracking cognitive and biomarker changes over time, the team found that participants with the highest ImP levels experienced significantly faster cognitive decline than peers with the lowest ImP levels.
Next, the team tested the effect of ImP in two groups of genetically modified mice engineered to develop Alzheimer’s-like pathology. Mice that received ImP over several months showed clear worsening of Alzheimer’s-related changes: some saw a marked increase in amyloid beta plaque buildup, while others developed more severe tau abnormalities and exacerbated dysfunction in astrocytes, the brain cells that support and protect neurons.
There Is No Single “Alzheimer’s Bacterium”
Notably, there is no single “Alzheimer’s-causing gut bacterium” linked to this process. Many of the bacterial strains that produce ImP are commonly found in the guts of healthy people. “ImP-producing bacteria are present in a large fraction of people, but they’re not very abundant in most people,” study co-author Federico Rey, a professor of bacteriology at the University of Wisconsin, said in a press statement. “But something we have learned over the years is that a microbe doesn’t have to be abundant to have an impact on the host.”
Simply having ImP-producing bacteria in the gut does not guarantee a person will develop high levels of the circulating metabolite, either. The amount of ImP that ends up entering the bloodstream depends on a complex combination of factors. The study found ImP levels vary by age, sex, and human genetics, while prior research has also linked ImP production to diet and overall gut microbiome composition. For the same reason, carrying ImP-producing bacteria does not mean a person will develop Alzheimer’s: age, genetics, metabolic health, vascular function, and many other factors all contribute to a person’s overall Alzheimer’s risk.
Could This Lead to a New Alzheimer’s Risk-Reduction Treatment?
At first glance, it might seem logical to develop therapies that target or eliminate ImP-producing gut bacteria to reduce Alzheimer’s risk, but the study authors warn the situation is far more complex. These bacteria produce ImP by metabolizing histidine, an essential amino acid that humans get from protein-rich foods and require for normal bodily function.
Instead of targeting the bacteria themselves, the researchers say a more promising path is to target ImP directly, or the specific bacterial biological pathways that produce it. The team compares this approach to the treatment of high cholesterol: just as statin drugs lower harmful LDL cholesterol to reduce heart disease risk, researchers could one day develop an inhibitor drug that lowers circulating ImP levels to cut Alzheimer’s disease risk.
This article was adapted from original reporting published in WIRED en Español, translated from Spanish.
