Major new piece of research --
Short and sweet:
The full 9 yards:
The middle article above contains an important qualification. In addition to bad gut microbes, you may need a genetic predisposition (or other adverse factor) to tip you over into PD:
"It is important to note that, in this study, gut microbes cooperate with a specific genetic factor to influence the risk for developing Parkinson's disease. The researchers used a specific genetic mouse model that recapitulates motor symptoms through α-synuclein accumulation, and genetically normal mice that were not predisposed to Parkinson's disease did not develop motor symptoms after receiving fecal transplants from patients. Other genetic and environmental factors, such as pesticide exposure, also play a role in the disease."