Why the brain’s microbiome could hold the key to curing Alzheimer’s

IT SEEMED like a classic case of Alzheimer’s disease. For three years, a man in his 70s had experienced serious cognitive decline: he frequently forgot the names of family members and was no longer able to drive or leave home by himself. Further deterioration seemed inevitable. But then his doctors checked a sample of his cerebrospinal fluid and noticed a fungus called Cryptococcus neoformans. When they put him on a course of antifungal medication, the results were startling. Within two years, he had regained his driving licence and returned to work as a gardener.

Neuroscientists have long suspected that certain infections can increase the risk of dementia. For instance, both Porphyromonas gingivalis, the bacteria behind gum disease, and the herpes simplex virus, which causes cold sores, have been linked with Alzheimer’s. But cases of “reversible dementia” are starting to inspire enormous interest in the idea that our brains are teeming with microorganisms – and that an imbalance in this “brain microbiome” may predispose people to neurodegenerative disease.

Until recently, the brain was thought to be devoid of microbes, not least because of the blood-brain barrier, a specialised membrane that keeps pathogens and toxins in the blood out of the brain. So the idea of a brain microbiome was controversial. However, a new study seems to clinch the case. Richard Lathe at the University of Edinburgh, UK, and his colleagues analysed data from post-mortem brains stored in four brain banks in the UK and US. They found a wide variety of microbes, with different types and…