What pathogens? If you have microbes that can use the plastic as a carbon source, they have a competitive advantage and will out-compete anything else.
Look up how ethanol fermentation is done -- it's actually not that sterile. The yeast secrete alcohol that effectively kills off competitors. A similar effect has been taken advantage of by engineering some yeasts to consume an unnatural source of phosphorus (phosphite).
So if you were to evolve a microbe that can eat certain types of plastic, it would readily outcompete everything else.