Environment

Except it doesn't hold up in the elements all that well, though (at least in a form that is still usable, the plastic is still there, just in little pieces and/or without the desired structural integrity). Plastics degrade when exposed to sunlight and oxygen (photo-oxidation). Combine that with mechanical action of waves, and now you have a bunch of little plastic bits floating in the ocean that are even harder to clean up (but easier for the bacteria to eat!). A glass or metal bottle will hold up much better than a plastic one, over a long enough time period.

But they even break down when exposed to temperature cycling and mechanical stress over long periods of time. I'm sure you've also noticed old plastic food containers, get harder and harder to clean and start getting cloudy: that's the plastic breaking down and micro-fissures appearing on the surface, thanks to repeated exposure to dishwashers, freezers, and still-hot leftovers. Once again, a glass dish is gonna hold up much better.

They have to use special additives for plastics intended for long-term outdoor use (the additives are like sunscreen for plastic, they absorb the UV so that the plastic doesn't) to combat these reaction pathways. And I'd bet money that if plastic-eating bacteria end up becoming a problem, there will be additives we can use to discourage them for appropriate applications.

But you'll notice that in the case of plastic in landfills, there's no UV light from the sun, basically no oxygen, and any mechanical stress or temperature cycling isn't enough for fast breakdown of the plastic polymers. These conditions are also very different from, say, your kitchen counter or hospital storage rooms. If the plastic-eating bacteria prefers the landfill habitat (or literally cannot thrive in any other environment -- which is not an uncommon phenomenon; in the article, they mention difficulties culturing bacteria for study in a lab environment), then we have a perfect tool for breaking down landfill plastics that won't impact in the slightest the plastics things we want to keep. Similarly, the kind of bacteria that could be useful for ridding us of fishing lines and nets floating around in the ocean would most likely not be well suited for non-aquatic environments.