New Scientist said a number of teams had been investigating Porphyromonas gingivalis, the bacteria behind chronic gum disease, and found that it attacks and inflames brain regions affected by Alzheimer's.
It said Alzheimer's cases often had accumulation of proteins called amyloid and tau in the brain, and the main hypothesis so far had been that defective control of these proteins caused the disease.
But since recent research had found people with amyloid plaques not suffering from Alzheimer's and efforts to treat the disease by cutting down on these proteins had failed, the hypothesis had come under serious scrutiny.
The report quoted Sim Singhrao of the University of Central Lancashire in the UK as saying: "This is the first report showing P. gingivalis DNA in human brains, and the associated gingipains, co-localising with plaques."
Singhrao, who has also conducted research into the cause of Alzheimer's, had earlier discovered that the bacteria invade the brains of mice which had gum infections.
Casey Lynch of Cortexyme was quoted as saying some brain samples with Alzheimer's showed the presence of the bacteria and the two proteins but at lower concentrations.
"We already know that amyloid and tau can accumulate in the brain for 10 to 20 years before Alzheimer’s symptoms begin'" Lynch said, adding that this showed the bacteria was a cause of Alzheimer’s, not a result.
"Alzheimer’s strikes people who accumulate gingipains and damage in the brain fast enough to develop symptoms during their lifetimes,” Lynch said. “We believe this is a universal hypothesis of pathogenesis.”
Cortexyme has developed a drug that block gingipains and intends to run a larger trial later this year.