Scientists at Harvard think it is possible. They injected Salmonella bacteria into the brains of mice. Overnight, amyloid plaques formed in the brains "and each plaque had a single bacterium at its center.” In contrast, mice that did not make amyloid succumbed to the bacterial infection.
"At this point, the Harvard researchers have what many say is an intriguing hypothesis, but they readily acknowledge that much work lies ahead. The Cure Alzheimer’s Fund is starting a large collaborative project that will use gene sequencing technology to carefully look for microbes in brains from people who had Alzheimer’s and those who did not. Researchers will also look for microbes in plaques found in human brains."