Back in the 70's I read a science fiction novel about a strain of bacteria that eats metal. The consequence to society was necessarily catastrophic. At the end of the novel scientists thought they had eradicated the bacteria but it merely mutated to an organism that subsisted on plastic.
I could have sworn the name of the novel was The Hephaestus Plague... but after a cursory Google search it seems that's not the book I remember.
In any case there are a multitude of science fiction horror stories about this particular Pandora's Box...
The manipulation of DNA through the auspices of technology and... hubris.

invented a bacterium that eats plastic eh? hmmm. well if true I can't help wondering what other amazing things this organism might do. how would it effect other life forms in the ocean that are critical to the balance of the oceans ecosystem. Might there be unintended negative consequences from introducing a new and artificially developed life form into the eco-system? Is it possible that this bacterium would eventually be more destructive to aquatic life than all the plastic material could ever be?