But, as always, this doesn't answer causation well.
Is there bacteria / fungus in the brain because the normal immune mechanisms in the brain are breaking as a consequence of AD and bacteria are happening to infect them, or is it the other way round.
The brain is not, as has been assumed in the past - a completely sterile organ.
There is immune activity going on in it in a carefully managed way, and barriers that are designed (*) carefully to keep stuff out.
Keeping the brain free of infection is a careful balance between allowing the right nutrients and cells in, and dealing with leakers carefully.
The massive dysfunction and structural changes in AD changing the blood-brain-barrier properties and the immune system reaction in the brain so it doesn't work as well would be entirely unsurprising.
(* not actually designed)
another brick in the wall of polymicrobial origin of Alzheimer (fungi and bacterias) published in 2017
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5514053/
"Several studies have advanced the idea that the etiology of Alzheimer’s disease (AD) could be microbial in origin. In the present study, we tested the possibility that polymicrobial infections exist in tissue from the entorhinal cortex/hippocampus region of patients with AD using immunohistochemistry (confocal laser scanning microscopy) and highly sensitive (nested) PCR. We found no evidence for expression of early (ICP0) or late (ICP5) proteins of herpes simplex virus type 1 (HSV-1) in brain sections. A polyclonal antibody against
Borrelia detected structures that appeared not related to spirochetes, but rather to fungi. These structures were not found with a monoclonal antibody. Also, Borrelia DNA was undetectable by nested PCR in the ten patients analyzed. By contrast, two independent Chlamydophila antibodies revealed several structures that resembled fungal cells and hyphae, and prokaryotic cells, but most probably were unrelated to
Chlamydophila spp. Finally, several structures that could belong to fungi or prokaryotes were detected using peptidoglycan and
Clostridium antibodies, and PCR analysis revealed the presence of several bacteria in frozen brain tissue from AD patients. Thus, our results show that polymicrobial infections consisting of fungi and bacteria can be revealed in brain tissue from AD patients."