Evidence supportive of a bacterial component in the etiology for Alzheimer's disease and for a temporal-spatial development of a pathogenic microbiome in the brain.

The AD-related pathogenicity of the brain microbiome seems to be based on a complex polymicrobial dynamic. The time ordering revealed a rise and fall of the abundance of with pathogenicity occurring for an off-peak abundance level in association with at least one other bacterium from a set of genera that included , , , , and . may also be involved with outcompeting the species, which were strongly associated with non-demented brain microbiota, whose early destruction could be the first stage of disease. Our results are also consistent with a leaky blood-brain barrier or lymphatic network that allows bacteria, viruses, fungi, or other pathogens to enter the brain.