Researchers at Columbia University analyzed 176 archived cerebrospinal fluid (CSF) samples from community-dwelling older adults — including many with Alzheimer's disease and related dementias — to test whether the CSF harbors a resident microbiome. Using 16S rRNA sequencing with spike-in bacterial controls to verify the method worked, they found almost no bacterial reads beyond what would be expected from contamination or sequencing error. A positive control from a bacterial meningitis patient correctly showed Streptococcus pneumoniae, validating the assay. The conclusion: CSF is genuinely sterile. Prior reports of oral bacteria like Porphyromonas gingivalis in CSF likely reflect contamination rather than a true microbiome, challenging a popular hypothesis linking oral bacteria directly to Alzheimer's pathology via the CSF.
