Mountains of plastic waste are a constantly growing environmental problem. There are also hardly any recycling models for some commonly used plastics. Plastic-decomposing bacteria, which are added to these polymers during production, could offer a future solution, a USA study shows.
A world without plastic is no longer conceivable. Plastic is used everywhere in beverage bottles, packaging material, clothing, crockery, floor coverings, window frames or furniture. It is light, inexpensive to produce and, above all, durable. This makes it a massive environmental problem, as plastic is difficult or impossible to break down in nature.
Some types of plastic are also almost impossible to recycle. Polyurethane (PU) plastic, widely used in industry due to its good stretch properties, was considered non-recyclable until a few years ago, precisely because of these characteristics. Although there are now some recycling methods, many products made from polyurethane—mobile phone covers, shoe soles, elastane in clothing, car parts or foam fillings for mattresses and upholstery (PU foam)—still end up in landfills or are incinerated at the end of their useful lives. Toxic substances can be released in the process.
Life in plastic
Repeated attempts have also been made to make these plastics biodegradable. However, many of these efforts deteriorated the desired material properties of polyurethane and were difficult to transfer to industrial processes.
According to a recent study published in the journal Nature Communications, a team led by Jonathan Pokorski and Han Sol Kim from the University of California, San Diego, has created a version of polyurethane enriched with living organisms that does not interfere with industrial production. The researchers used spores of the bacterium Bacillus subitilis, which they added to the plastic during manufacturing.
Hunger for plastic
Bacillus subtilis is one of the best-known spore-forming bacteria. Some types of bacteria have developed spores to cope with potentially hostile conditions such as high temperatures, high pressure and toxic substances such as acids, bases and solvents. These spores are inactive for most of their lives and can remain dormant for years, only to germinate and transform into viable cells within minutes under the control of external triggers.
The scientists wished to make use of precisely these properties. Another advantage of Bacillus subtilis is that some strains of this bacterial species, found almost everywhere in nature, can decompose polyester-based polymers such as polyurethane – in other words, they feed on plastic.
Heat is the problem
However, their spores cannot tolerate the high temperatures of more than 130 degrees Celsius necessary in the industrial production of polyurethanes. They can only survive for a few minutes at temperatures around 100 degrees Celsius.
Using biotechnological processes such as adaptive laboratory evolution (ALE), the researchers produced heat-resistant spores that can tolerate significantly higher temperatures. These spores were then added to the plastic as living additives. Almost 100 percent of all biotechnologically modified spores survived the processing temperature of around 135 degrees Celsius required for producing thermoplastic polyurethane.
Bacteria as waste workers
To test whether and how well the enriched plastic can decompose in nutrient-poor landfill soils, the research team produced sterilised compost to which only a few microorganisms were added. Even in this microbe-poor environment, the spores were able to germinate. Within five months, around 93 percent of the polyurethane plastic was decomposed by the bacteria at an ambient temperature of 37 degrees Celsius.
However, the study leaves the question of whether toxins can escape from the polyurethanes into the soil during this decomposition process. ‘We are still trying to understand which substances are released during the decomposition process. So far, however, we have found no evidence of toxic pollutants,’ Pokorski explains to science.ORF.at. It is also not yet fully understood whether the plastic is actually broken down by the bacteria or only decomposes. ‘However, we have already established that CO2 is produced during this process,’ says Pokorski. In any case, both need to be investigated in further studies.
- source: orf.at/picture: Bild von Pixabay
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