Brains of deceased dementia patients were found to contain signs of Porphyromonas gingivalis, the bug causing unhealthy gums.
Scientists believe when the bacteria reach the brain they trigger an immune response that can lead to the death of neurons.
This could help drive the changes that are typical of Alzheimer's disease, causing confusion and memory loss.
Scientists at the University of Central Lancashire studied 10 brain samples from patients who died with dementia. They were compared with the same number of samples from non-sufferers. Professor StJohn Crean, dean of the School of Medicine & Dentistry at the university, said: "It remains to be proven whether poor dental hygiene can lead to dementia in healthy people, which could have significant implications for the population as a whole. These bacteria could make the existing disease condition worse."
The findings are reported in the Journal of Alzheimer's Disease.
Lead researcher Dr Sim Singhrao said: "We are working on the theory when the brain is repeatedly exposed to bacteria and/or their debris from our gums, subsequent immune responses may lead to nerve cell death and possibly memory loss."