I've been doing a bit more reading ...

It has been established that gut microbes differ between PWPs and healthy controls [1]. It has also been established that PWPs exhibit chronic intestinal inflammation [2]. It has also been established that peripheral (e.g. intestinal) inflammation produces neuroinflammation [3].

There is a theory that links these together. In PWPs, gut microbes somehow cause chronic intestinal inflammation, which causes chronic neuroinflammation. Chronic neuroinflammation then causes (among other things) the c-Abl kinase to become active in fully developed neurons (where it is otherwise relatively quiescent). The c-Abl kinase (among other things) then prevents the Parkin protein from performing its normal function, etc. etc.

Why do the gut microbes differ between PWPs and healthy controls? In my case it may be related to the fact that, as a child, I spent a couple of weeks or more in hospital being treated for rheumatic fever. For the next ten years I was given antibiotics daily. I can only imagine what degree of dysbiosis this caused to my microbiome! I remember that Blackfeather told a similar story a few months ago, involving broad spectrum antibiotics.

[1] Short chain fatty acids and gut microbiota differ between patients with Parkinson's disease and age-matched controls, Marcus M. Unger et al. Parkinsonism & Related Disorders, November 2016.
[2] Colonic inflammation in Parkinson's disease, David Devos et al. Neurobiology of Disease, February 2013.
[3] Consequences of the Inflamed Brain, Steven F. Maier, Linda R. Watkins, Dana Alliance Report on Progress, August 2012.