The plastic in your body might be _less_ toxic than paracetamol.

The major part of this discovery was about something that e. coli already did on its own before genetic modification, so no not particularly. They might be able to engineer a symbiotic bacterium to live in your blood (there's a lot of doubt here), or more likely and less risky engineer a bacterium that consumed microplastics to live in your gut delivered as a probiotic or as a "vaccine" that you ate that would infect your existing microbiome.

And not an mRNA vaccine, those are kind of too simple and are mainly there to annoy the immune system. You would either use a virus to inject new genes into bacteria or insert plasmids into bacteria in a lab which would later be consumed.