For decades, scientists searching for the origins of devastating neurological illnesses such as Alzheimer’s disease and Parkinson’s disease have focused almost entirely on the brain. But a new study suggests the story may begin much earlier — in the gut.

Researchers from Emory University in the United States have reported evidence that bacteria from the gut can travel to the brain under certain conditions, raising the possibility that this “backdoor route” could play a role in neurological diseases. The results were just published in the journal PLOS Biology.

The research team found that mice that ate a lot of fat developed gut dysbiosis, which is an imbalance in the community of bacteria that live in the digestive system. There are trillions of beneficial bacteria in the human stomach that help with digestion and immunity. However, disrupting this balance can lead to negative consequences.

The high-fat diet changed the types of microorganisms in the stomach and made the gut more permeable, which is what scientists call a "leaky gut." This means that the intestinal barrier is less effective in keeping microbes from escaping the digestive system.

Under these conditions, researchers discovered that a small number of bacteria moved from the gut into the brain of the mice. Scientists believe this migration may occur through the vagus nerve, a major communication pathway linking the digestive system and the brain.

Importantly, when the mice were switched back to a normal diet, the bacteria detected in their brains disappeared. This suggests the process may be reversible, though researchers caution that the experiments were conducted only in animals.

The team also found low levels of bacteria in the brains of mouse models used to study neurological conditions including autism spectrum disorder, Alzheimer’s disease and Parkinson’s disease — even when the animals were not exposed to a high-fat diet.

Scientists have long known that the gut and brain communicate through what researchers call the “gut–brain axis”, a complex network involving immune signals, hormones and chemical compounds produced by microbes. However, the direct movement of bacteria into the brain has remained poorly understood.

The new findings add to growing scientific evidence suggesting that disturbances in gut microbes may influence neurological health. Around the world, the number of people living with neurodegenerative diseases is rising sharply as populations age, intensifying the search for early triggers and prevention strategies.

“While the incidence of multiple neurological conditions is increasing, the initiating causes are largely unknown. This novel pathway of gut bacteria reaching the brain could be a trigger of numerous neurological diseases,” the study authors said.

Before drawing conclusions about humans, experts caution that much more research is necessary. Mouse models are often used to explore biological mechanisms, but results do not always translate directly to people.

Even so, the discovery is likely to intensify scientific interest in the gut microbiome—the vast ecosystem of microbes inside the human digestive tract— and its possible role in brain health.

If future studies confirm that bacteria can reach the brain through similar pathways in humans, scientists say it could reshape how researchers understand the early stages of neurological disorders and potentially provide novel opportunities for prevention, diet-based therapies, and microbiome-targeted treatments.