One of several reasons I don't like French supermarkets is because they are so miserable with plastic bags at the checkout, which always catches me unprepared. Bags are handed out like precious heirlooms and I usually end up with groceries I would distribute over three or four bags here, all packed into one bag, making the carrying handles stretch into thin cords that bite into my fingers. The idea is to reduce plastic waste, which is, of course, laudable. If I lived in France, I would shop pre-prepared with my own re-usable cloth bags. We will all soon have to adopt this habit in Ireland, as the Government intends to make us pay 10p for every plastic bag we take in the supermarket.
Plastic is the most obvious product of modern chemistry in the everyday world. Plastics comprise a large number of synthetic organic materials that are moulded when soft and set into any of a huge varieties of shapes. Plastics are an enormously versatile class of material and can be manufactured with properties ranging from soft and pliable to hard and strong. Plastic meets the eye everywhere you look - pipes, windows, chairs, pens, computers, refrigerators, TV, radios, etc.
One very valuable property of plastic is that it is very durable - it doesn't rust or corrode. However, this very property also causes the single biggest problem we have with plastic. We design plastic to be indestructible and then we use it to make disposable goods; bottles, biros, cups and cutlery, bags, etc. Plastic is not a natural product and nature is unable to break it down, making it a permanent pollutant and disrupter of the environment.
There are three possible solutions to plastic waste: (a) stop using plastics altogether; (b) recycle all discarded plastic products and (c) develop new forms of bacteria capable of breaking down used plastic to harmless products.
Solution (a) is simply not a runner. Our modern world is far too dependent on the flexibility of plastic to change away from it at this stage. Solution (b) could solve the waste problem but it is practised on a small scale only. For example, every year, 20 million tons of plastics are thrown away in America, but only 10 per cent of this is recycled. We discard 1.2 billion plastic bags in Ireland every year. Recycling is seen as an expensive solution, but surely this is true only in the economics of the short term.
Solution (c) is the main subject of my article. All discarded natural products eventually disappear because they are digested by nature. We make active use of this principle in the way we deal with certain waste products. Each house in country areas has its own septic tank into which the sewage is piped to be digested away by naturally occurring bacteria. In towns and cities sewage is piped into treatment tanks where it is mixed with micro-organisms to form an `activated sludge'. Organic-rich waste water from food and other industries can be likewise dealt with in special treatment plants.
Micro-organisms, particularly bacteria, are very flexible and adaptable and there is scarcely any natural product that they cannot digest. However, micro-organisms did not encounter artificial synthetic chemical products over their evolutionary history and consequently they didn't develop the capacity to digest such material. Nevertheless, so flexible are these creatures that in recent decades some micro-organisms have evolved a capacity to digest a range of synthetic pesticides, solvents and refrigerants. If we could tailor-make bacteria to consume plastics they could be naturally broken down into eco-friendly by-products.
Carbon tetrachloride is a widely used toxic chemical (it will cause liver damage if inhaled in excessive amounts). For example, it is used in dry cleaning, extinguishing fires, fumigating grain silos, etc. Because of its widespread use, there is an ongoing risk that it will leach into waterways. A bacterium (Pseudomonas stuzeri) has been found that is very good at breaking down carbon tetrachloride and it has been successfully used to clean up spills of carbon tetrachloride from grain silos.
Intensive efforts are ongoing worldwide to identify naturally occurring bacteria and to develop new bacteria, capable of breaking down synthetic toxic wastes. In order to do this it is necessary to understand the basic mechanisms whereby micro-organisms naturally develop the capacity to destroy synthetic toxins. Classical genetics explains this evolution of new capacity in terms of an accumulation of random changes (point mutations) in genes allowing them to carry out new functions (i.e. destroy the new toxin). However, other mechanisms, possibly more important, also seem to be at work.
Studies have shown that micro-organisms can develop the capacity to degrade new synthetic chemicals by shuffling existing genes amongst different varieties of themselves, as well as by evolving new genes to do the job. In other words, the necessary genes are already there, but have been doing other jobs. When they are shuffled into partnerships with other appropriate genes it can become possible to degrade the new synthetic toxin. It is hoped that new strains of bacteria capable of degrading synthetic chemicals can be developed by challenging mixtures of bacteria repeatedly with the chemicals. This process will be helped along as necessary by genetic engineering, i.e. inserting new genes into the bacteria.
Much of our modern technology is made of plastic. Could new plastic-eating superbugs get free and eat up all the technology of which we are so proud? The experts emphatically dismiss these fears. The new bacteria, they say, will be dependent for survival on a specialised environment, outside of which they will die. They will be unable to go wild. Anyway, the experts point out, paper and cotton are biodegradable, yet neither our books nor our shirts disappear before our eyes.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.
