Table 1.

Summary of recent findings on brain tissue microbiome.

Sample type	Source	Methods	Findings	References
Brain tissue	four different CNS regions of one AD patient and entorhinal/cortex hippocampus samples from an additional eight AD patients	DNA sequencing	Botrytis cinerea and Cryptococcus curvatus are common to all four CNS regions. Five genera are common to all nine patients: Alternaria, Botrytis, Candida, Cladosporium, and Malassezia.	(20)
	gray and white matter were studied from 24 AD patients and 18 age-matched controls	DNA sequencing	E coli K99 and LPS levels are greater in AD compared to control brains. Gram-negative bacterial molecules are associated with AD neuropathology.	(21)
	14 AD patients and 12 matched controls	16S rRNA	AD brains tend to have strikingly large bacterial loads compared to controls	(22)
	Postmortem hippocampal formation specimens from 10 neurological controls,10 AD patients, 22 AD patients and 19 neurological controls from the hippocampus, 12 control and 20 AD cerebellum samples	16S rRNA	Independent of study in both AD and control subjects the most abundant phyla were Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Variations in beta diversity between hippocampal and cerebellum samples were observed indicating an impact of brain region on the presence of microbial DNA.	(23)
	10 AD patients and 7 matched controls	16S rRNA	PCR analysis revealed the presence of several bacteria in frozen brain tissue from AD patients. Results show that polymicrobial infections consisting of fungi and bacteria can be revealed in brain tissue from AD patients.	(24)
	711 AD and non-AD control brains	RNA-seq and whole-genome sequencing	HHV-6 demonstrated little specificity to AD brains over controls by either method, while other viruses such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV) were detected at comparable levels. These direct methods of viral detection do not suggest an association between HHV-6 and AD.	(25)
Breast, Lung, Ovary, Pancreas, Melanoma, Bone, and Brain	1526 tumors and their adjacent normal tissues samples across seven cancer types. 643 negative controls	16S rRNA and histological staining methods	Each tumor type has a distinct microbiome composition. The intratumor bacteria are mostly intracellular and are present in both cancer and immune cells.	(11)
Glioma	3 human glioma samples	Tissue clearing, immunofluorescent labeling, optical sectioning microscopy,	the presence of microbiota in human gliomas	(12)