But before we get into the articles about the gut microbiome, from 2015, a review of the various signs that predate full motor symptoms

non-motor and pre-motor symptoms of PD

Recent studies provide new insights on the frequency, pathophysiology, and importance of non-motor features in Parkinson’s disease as well as the recognition that these non-motor symptoms occur in premotor, early, and later phases of Parkinson’s disease.

This study (2017) looks at the progression of PD and problems with the bacteria in the stomach:
Progression of PD and gut dysbiosis
from the conclusions:

The total counts of intestinal bacterial decrease in the course of PD progression. Temporal profiles of lowering of bacterial counts are likely to be different from bacteria to bacteria, and also between the deteriorating and stable groups, which may be able to be exploited to differentiate patients with rapidly and slowly progressive PD pathology.

The search for biomarkers for PD goes for the gut (2018):
Gut Microbiota Dysfunction as Reliable Non-invasive Early Diagnostic Biomarkers in the Pathophysiology of Parkinson’s Disease: A Critical Review

future directions should therefore, focus on the exploration of newer investigational techniques to identify these reliable early biomarkers and define the specific gut microbes that contribute to the development of Parkinson’s disease. This ultimately should pave the way to safer and novel therapeutic approaches that avoid the complications of the drugs delivered today to the brain of Parkinson’s disease patients.

Another article on the role of gut bacteria in PD (2018): Parkinson’s disease and bacteriophages as its overlooked contributors

Our findings add bacteriophages to the list of possible factors associated with the development of PD, suggesting that gut phagobiota composition may serve as a diagnostic tool as well as a target for therapeutic intervention, which should be confirmed in further studies. Our results open a discussion on the role of environmental phages and phagobiota composition in health and disease.

A review of the relationship between the gut microbiome and PD (2018):
Microbes Tickling Your Tummy: the Importance of the Gut-Brain Axis in Parkinson’s Disease

Looking back a few years, one can hope that diet might be a way to affect the course of PD, if diet can change the gut microbiome (2014) :Diet rapidly and reproducibly alters the human gut microbiome

Long-term diet influences the structure and activity of the trillions of microorganisms residing in the human gut1–5, but it remains unclear how rapidly and reproducibly the human gut microbiome responds to short-term macronutrient change. Here, we show that the short-term consumption of diets composed entirely of animal or plant products alters microbial community structure and overwhelms inter-individual differences in microbial gene expression.

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