The real answer is "no one knows", but my intuition would be that you wouldn't expect them to be particularly harmful. One of the advantages of consuming a nov

The real answer is "no one knows", but my intuition would be that you wouldn't expect them to be particularly harmful. One of the advantages of consuming a novel food source like plastic is a relative lack of competition; this means you wouldn't expect there to be a lot of benefit for producing toxins to kill off competitors for your foodstuff. (For an analogous case, the microorganisms that can grow on maple syrup don't tend to produce toxins; there's not much of a need when their food will kill most bacteria.)