Plastic waste is polluting the environment worldwide. One hope for combating the waste problem could be so-called plastic-eating bacteria. These are able to feed on the carbon compounds from the plastics and thereby biodegrade the plastic. Two studies have now examined the metabolism of such bacteria in more detail and explored options for cultivating the plastic eaters. The results pave the way to industrial use of the biological helpers.
Plastic offers us numerous useful properties: it is light, inexpensive, shatterproof, versatile and durable. However, longevity becomes a problem when it comes to disposal. While some plastic products can be recycled, they often accumulate in the environment instead, where they can last for centuries. Especially for materials that consist of different types of plastics, recycling has so far been very complex or hardly possible. These include, for example, products that are coated with the plastic polyester urethane (PEU), including fishing nets, ropes and certain textiles. PEU increases the shelf life of the products, but at the same time makes them more difficult to decompose.
Compost bacterium with special properties
A research team led by Jan de Witt from the Jülich Research Center has now taken a closer look at a genus of bacteria that could provide a solution. The so-called Halopseudomonas bacteria occur in extreme habitats, including areas of the deep sea that are polluted with petroleum or heavy metals. The species that de Witt and his team were working on, Halopseudomonas formosensis FZJ, was isolated by the researchers from a German compost. Their experiments showed that these bacteria can also break down the hydrocarbon structure of some plastics.
“The bacterium is able to grow on different types of PEU and use this plastic as its sole carbon source,” report de Witt and his team. “After 72 hours of cultivation, various PEU coatings were completely depolymerized.” In addition to the high rate of decomposition, the researchers discovered an important advantage of the strain they isolated over other Halopseudomonas species: “The strain we isolated was particularly tolerant to high temperatures and was able to degrade the plastic at up to 50 degrees Celsius,” said the team. “Most other species only grow at temperatures of up to 37 degrees Celsius.” This property is important for possible industrial applications, which often involve high temperatures – similar to the inside of a compost heap, the natural habitat of H. formosensis.
On the way to biotechnological application
Using genetic analysis, the research team discovered an enzyme produced by the bacterium that is important for breaking down the plastic. If they switched off the gene for this enzyme, the bacteria genetically modified in this way could hardly break down the PEU. This result proves, on the one hand, the crucial role of the identified enzyme and, on the other hand, shows that it is possible to genetically modify H. formosensis. For future applications, it would also be conceivable to increase enzyme production and thus the activity in plastic degradation using genetic engineering.
An important step on the way to the biotechnological application of bacteria is to make them cultivable on a large scale. This was the subject of a second study by researchers from the same team, led by Luzie Kruse from Heinrich Heine University Düsseldorf. “These bacteria cannot yet be easily cultivated using existing cultivation and cloning methods,” explain the researchers. “We have addressed these limitations and established microbiological and molecular genetic methods for several Halopseudomonas strains.” These methods can serve as a kind of toolbox in the future and pave the way to actually using Halopseudomonas industrially.
“The profound knowledge of microorganisms and enzymes contributes to the understanding of the interaction of microbes with synthetic polymers,” write the researchers. “Therefore, this study points the way for future bio-recycling strategies aimed at recycling whole plastics, including their coatings.”
Sources: Jan de Witt (Jülich Research Center) et al., Microbial Biotechnology, doi: 10.1111/1751-7915.14362; Luzie Kruse (Heinrich Heine University Düsseldorf) et al., Microbial Biotechnology, doi: 10.1111/1751-7915.14369