The bacteria also boosted production of amyloid beta, a component of brain plaques associated with Alzheimer's, BBC News reported.
Further tests on mice showed that drugs that block the toxic proteins produced by the bacteria stopped brain degeneration.
The study was published in the journal Science Advances.
The researchers said their findings could point to new ways to help people with Alzheimer's. Currently there is no cure or effective treatment, BBC News reported.
The team developed a new drug and plan to test it later this year in a clinical trial with patients who have mild to moderate Alzheimer's.
The study adds to evidence of a link between gum disease and dementia, but it's still not clear if gum disease bacteria actually trigger Alzheimer's, said scientists not involved in the study, BBC News reported.
Previous studies linking gum disease with dementia include one published last year that found that people with chronic gum disease for 10 years or more had a 70 percent higher risk of Alzheimer's than those without gum disease.