Jan 24 2023

Bacteria That Eat Plastic

The world produces about 380 million tons of plastic every year, and half of that is single use plastic. This figure is projected to increase by 70% by 2050. A 2017 study found that of all the plastic produced, “9% of which had been recycled, 12% was incinerated, and 79% was accumulated in landfills or the natural environment.” Current practices are unsustainable, as a lot of this plastic ends up in the oceans and elsewhere in the environment. Researchers are increasing looking to bacteria as one potential solution to this problem.

Recycling alone is not a viable solution. Currently, the main effect of recycling plastic is to create a false sense that the problem is being addressed. Plastic is not really recycled in a full circular process. A relatively small portion of plastic is broken down and then remanufactured into lower quality plastics that cannot be recycled. The ultimate fate of all plastics is still the incinerator (not good for the environment), landfills, or the oceans and other environmental locations. What are the potential solutions to this situation?

One solution is to use less plastic. But this is a complex suggestion. Single use plastic is the most common target, and there is a lot of wasteful use that certainly can be reduced. But half of plastic produced is not single use, and a lot of plastic is necessary for certain applications (such as sterile medical use). As always, you also have to consider the alternatives – what will replace this plastic? Sometimes there is an obvious and good solution. In many beverage applications, aluminum can replace plastic and is more recyclable.  Glass is another alternative material. But this approach will only get us so far.

Part of the problem is that plastic is cheap to mass produce, and it works very well for the intended applications. It’s cheaper to make virgin plastic than to recycle old plastic. This reality feeds the problem.

This is where bacteria come in. I will say at the outset that there is likely no simple solution here, such as letting lose a plastic-eating bacteria that makes all the plastic waste go away. But bacteria, and specifically their enzymes, can be leveraged to move us towards a more circular plastic economy, or to reduce the environmental burden of plastic. There are a few pieces of good news here. First, many bacteria have already evolved enzymes to break down plant fibers that are similar to some kinds of plastic. Further, bacteria appear to be evolving to eat environmental plastic. If you supply a potential massive food source, something will likely evolve to eat it. A 2021 study found that bacteria with enzymes that can break down plastic are thriving in areas where there is more plastic.

Another recent study found that bacteria really do eat plastic, meaning that they metabolize constituents in the plastic for food. They used plastic made with carbon 13 and found that C13 in CO2 released by the breakdown of plastic in the presence of bacteria (Rhodococcus ruber). Incidentally, this study also found that the bacteria could break down at least 1% of the plastic available to them per year. That’s not a lot, but it’s not nothing.  A survey of ocean and soil bacteria found a total of 30,000 different bacterial enzymes capable of breaking down many different kinds of plastic. That’s a lot to work with.

In addition to using bacteria that already have plastic-eating enzymes, researchers are working on genetically engineering bacteria that contain either a suite of such enzymes or other properties that make them ideal for certain applications. Once we have the ideal bacteria in hand, what do we do with them?

This is always the tricky part – going beyond proof of concept to designing an industrial scale practical and economical application. On the “economical” side, that often comes down to, how much are we willing to spend, and what do we value. This, I believe, is also yet another example of an externalized cost, allowing companies to simply not take responsibility for the cost of their products ending up as pollution in the environment. If we collectively value plastic-free oceans, then we should be willing to pay a bit more for plastic that won’t end up there. But still – the cheaper we can make any such process using bacteria the better. Can we scale it?

Bacteria have a couple of properties that do make them scalable and cost-effective. They are self-reproducing, and they use relatively little energy. In fact, if they actually eat plastic the plastic itself becomes part of the energy driving the process. Another source of energy is UV light from the sun, which also breaks down plastics into smaller bits and makes them easier for bacteria to digest. Sunlight and bacteria seems potentially scalable.

One use of such bacteria is in a factory setting. Recycled plastic can be subjected to such bacteria that will break the plastic down into constituent molecules that can then be used as raw material to remanufacture the same kind of plastic. This can create real recycling with a truly circular plastic lifecycle. Obviously there will have to be some waste in the process, so some virgin raw material will still need to be fed into the system, but the idea is to minimize that. Moving towards maximizing circular plastic production may also require some tweaking to the kinds of plastics that are used in different applications, relying more on those plastics that are most amenable to this process.

Another application is to use plastic consuming bacteria to reduce plastic in landfills or other contained situations. One advantage to this application is that bacteria can be applied to mixed plastic products, those that contain different kinds of plastic. These are really hard to recycle, but bacteria can just eat the parts they can eat and leave the rest. The downside to this application is that it is a kind of slow burn – it still releases CO2 into the atmosphere. But this can be easily combined with CO2 capture technology (such as systems already in place to capture methane from organic breakdown in landfills).

Yet another potential application is the release of engineered bacteria into the environment (such as the oceans) to eat environmental plastic waste. I don’t see this happening anytime soon, primarily because of appropriate caution around releasing engineered bacteria into the wild. But I wouldn’t rule this out either, especially if scientists just tweak an existing bacterial species that already exists in the wild and is eating plastic.

What’s clear is that plastic-eating bacteria is not a simple or single magic solution to the plastic waste problem. But it can become a critical part of an integrated set of solutions.

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