It depends on the type of plastic. No microbes are known that can degrade styrofoam. That's why we have no image here. This microbes makes a plastic which will degrade if put on a compost heap. This plastic is called polyhydroxybutyrate, or PHB for short.
Some plastics are biodegradable.
Biodegradable means that organisms can degrade the material. This is why when you put a Styrofoam cup on your compost pile it does not turn into compost, but instead just sits there. Build a compost pile in your backyard. Observe it every couple of weeks and record your observations.