Contrary to what is implied in your article (The brain microbiome: could changing it help prevent dementia?, December 1), the weight of expert opinion in the field of medical microbiology rejects the existence of a ‘brain microbiome’ in the sense of a local microbial community. in healthy human brains. While pathogenic microbes – such as Borrelia burgdorferi, that causes Lyme disease, or fungal pathogens such as Cryptococcus neoformans – can enter the brain and cause neurological symptoms; these are examples of infections, not evidence of a native microbial community.
Similarly, the 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 been independently validated nor do they provide any consistent picture. This mirrors the controversies surrounding other putative microbiomes – such as that of the placenta – that have failed to withstand independent scrutiny. More than a decade of research suggests that contamination, usually from laboratory reagents, is the most plausible explanation for such findings, especially when even so-called ultrapure water has been shown to harbor DNA signatures and culturable microbes.
If diverse microbes are truly abundant in the brain, why have they not been cultured repeatedly and consistently over the century and a half or so of medical microbiology? Why have they not been observed in numerous microscopy studies of human brain tissue? Efforts to investigate the overlooked role of microbes in neurological disorders are welcome, but they must be based on robust and reproducible science – not on a speculative discussion of a ‘brain microbiome’.
Prof. Mark Pallen and Dr. Aimee Parker Quadram Institute, Norwich, Prof. Nick Loman University of Birmingham, Professor Alan Walker University of Aberdeen