Contrary to what is implied in your article (The brain microbiome: could understanding it help prevent dementia?, 1 December), the weight of expert opinion in medical microbiology rejects the existence of a “brain microbiome” in the sense of a resident microbial community in healthy human brains. While pathogenic microbes – such as Borrelia burgdorferi, which causes Lyme disease, or fungal pathogens like Cryptococcus neoformans – can invade the brain and cause neurological symptoms, these are examples of infections, not evidence of a native microbial community.
Similarly, cognitive benefits of vaccines can be explained by their role in preventing infections or modulating immune responses and inflammation, rather than any impact on a “brain microbiome”. Furthermore, the studies cited in the article have not undergone independent validation, nor do they provide any kind of consistent picture. This mirrors controversies around other supposed microbiomes – such as that of the placenta – which have failed to withstand independent scrutiny. Over a decade of research indicates that contamination, typically from laboratory reagents, is the most plausible explanation for such findings, particularly when even supposedly ultrapure water has been shown to harbour DNA signatures and culturable microbes.
If diverse microbes are truly abundant in the brain, why have they not been repeatedly and consistently cultured in over a century and a half of medical microbiology? Why have they not been observed in numerous microscopy studies of human brain tissue? Efforts to explore overlooked roles of microbes in neurological conditions are welcome, but they must be grounded in robust and reproducible science – not speculative discussion of a “brain microbiome”.
Prof Mark Pallen and Dr Aimee Parker Quadram Institute, Norwich, Prof Nick Loman University of Birmingham, Prof Alan Walker University of Aberdeen
