To attempt to rule out the possibility that the brain samples were contaminated, the researchers analyzed mouse brains that were preserved immediately after death. The scientists also found "abundant bacteria" in the mouse brains, and the bacteria were in similar locations to those in human brains, according to the abstract. And when the researchers analyzed "germ-free" mice, which are genetically engineered to not have any bacteria living in them, the scientists did not find any bacteria in the brains.