How does plastic 'break down'?

Decomposition is part of the nutrient cycle and it's the process by which bacteria or fungi break down organic substances into simpler matter.
Traditional plastic, much known as polyethylene terephthalate, is composed of many chemicals that bacteria cannot consume or break down. Polyethylene terephthalate (PET or PETE) is a general-purpose linear semi-crystalline thermoplastic polymer. When bacteria cannot breakdown, or decompose certain substances Vast reservoirs of organic compounds that cannot be broken down without the aid of enzymes found only in bacteria would eventually accumulate, and essential components of the ecosystem would become irrecoverably locked in place. Now, what does this all mean? In more simple terms: When plastic cannot break down, or decompose, the plastic simply does not; The plastic remains the same substance up until 450 centuries later.