The researchers found that mice suffering from Alzheimer's have a different composition of intestinal bacteria compared to mice that are healthy.
Mice without bacteria had a significantly smaller amount of beta-amyloid plaque - lumps that form at the nerve fibres in cases of Alzheimer's disease - in the brain.
"Our study is unique as it shows a direct causal link between gut bacteria and Alzheimer's disease. It was striking that the mice which completely lacked bacteria developed much less plaque in the brain," said Frida Fak Hallenius from the Lund University in Sweden.
"The results mean that we can now begin researching ways to prevent the disease and delay the onset," Hallenius added.
Gut bacteria has a major impact on how people feel through the interaction between the immune system, the intestinal mucosa and our diet.
The composition of the gut microbiota depends on which bacteria we receive at birth, our genes and our diet, the researchers said.
In the study, the team also studied Alzheimer's disease in mice that completely lacked bacteria to further test the relationship between intestinal bacteria and the disease.
They transferred intestinal bacteria from diseased mice to germ-free mice.
The mice developed more beta-amyloid plaques in the brain compared to if they had received bacteria from healthy mice, the researchers noted.