Evolution in process

Now that humanity has made plastic as much a part of the Earth's environment as wood and leaves, it shouldn't really come as a surprise that one day bacteria will evolve (or humanity will help bacteria to evolve) to degrade plastic. Then we can start wondering whether that will be a helpful thing or result in a nightmare scenario. Probably somewhere in between, as always. Decomposition could work fast, but not that fast. Plastic might eventually become a more naturally-destructible material instead of something considered almost absolutely resistant to the elements; but then again, how many instances of mass chaos caused by wood-decomposing bacteria have there been in the past?