The prodromal microbiome

In human microbiome research, reproducibility is crucial because the microbial ecosystem is heavily influenced by environmental factors and highly variable between individuals. This poses challenges when designing such studies and recruiting suitable subjects. Furthermore, the employed methodology of data acquisition and analysis can be highly customized, and differences in the work flow may introduce biases and hamper comparisons between studies. In the last 2 years, the gut microbiome has gained increasing attention in the Parkinson’s disease research community because of its possible implication in the prominent gastrointestinal disturbances frequently present from the earliest stages of PD and the ascension hypothesis of PD-related neuropathology. So far, 9 original publications have reported gut microbiota alterations in PD. As could be expected, the subject selection criteria, laboratory protocols, and bioinformatical and statistical pipelines differed considerably between studies. Sample size ranged from 38 to 327 subjects. Most studies used 16S ribosomal RNA amplicon sequencing to assess the community structure of the gut microbiota, whereas 2 studies relied on qPCR analysis of preselected bacterial taxa. These approaches are relatively cost effective and allow a description of differences in the relative abundances of bacterial taxa between study groups (ie, the community structure). Despite the above-mentioned limitations and considerable differences in their methodology and results, it is encouraging that the abovementioned studies showed clear overlaps with respect to the bacterial taxa differentially abundant between PD patients and control subjects. Increased abundance of Akkermansia and Lactobacillus and decreased abundance of Prevotella in PD subjects were reported in 5 studies. Other reproducible findings were reduced abundance of Faecalibacterium (4 studies) and Blautia (3 studies) and increased abundance of Bifidobacterium (3 studies) in PD. So by now, that the gut microbial community structures of PD patients and controls differ seems to be reasonably well established, with several bacterial genera showing consistent abundance alterations in multiple studies. However, it was not known whether these alterations precede the development of PD or whether they are rather a consequence of a dysfunctional gut associated with PD. In this issue of Movement Disorders, HeintzBuschart et al add an important piece of evidence to the field by demonstrating that indeed gut microbiome alterations may precede the motor symptoms of PD. In addition to patients with established PD, they studied a group of subjects suffering from a well-defined prodromal syndrome of PD, namely, rapid eye movement sleep behavior disorder (RBD). Because RBD is associated with an up to 90% risk of developing a neurodegenerative disorder later on (mostly synucleinopathies), it is considered a promising criterion for selection of patients for premotor trials of diseasemodifying treatments. In their study, Heintz-Buschart et al employed stateof-the-art analytical pipelines to assess microbiome community structure in nasal and fecal samples and aimed to account for potential confounders such as constipation and medications. The rationale for the analysis of nasal microbiota was that, like the gastrointestinal tract, the nasal cavity has been suggested to constitute a port of entry for a possible pathogenic agent. For the first time, in this study, nasal microbial communities were directly compared with fecal communities. As has been previously described, the nasal bacterial community was sparse and showed high interindividual variation. Operational taxonomic units (OTUs) common to the nasal and gastrointestinal microbiota were found, likely attributable to nasal discharge being swallowed. However, none of these OTUs differed significantly between study groups. *Correspondence to: Filip Scheperjans, Department of Neurology, Helsinki University Central Hospital, Haartmaninkatu 4, 00290 Helsinki, Finland; filip.scheperjans@hus.fi