Plastic waste presents a challenge, yet it also offers a chance to turn waste into valuable materials for society. Researchers at Rensselaer Polytechnic Institute in the United States have developed a strain of bacteria that can convert plastic waste into biodegradable silk with multiple applications.
The results were published in Microbial Cell Factories. This work marks the first instance of a team guiding a bacterium to transform polyethylene into a silk-like material. Polyethylene is a common component of disposable items, including bags and packaging, and the new approach aims to produce a high-value protein product from it.
This material, described by the researchers as a biology-inspired spider web, bears similarity to the silk spiders use to weave their threads. It has potential uses in textiles, cosmetics, and even medicine.
“Spider webs are nature’s Kevlar”, stated Helen Zha, a professor of chemical and biological engineering and a leading investigator on the project. “It could be nearly as strong as steel, yet its density is six times lower, making it very light. As a bioplastic, it is elastic, strong, non-toxic and biodegradable.”
All of these properties point to the material being ideal for a future focused on renewable resources and reducing persistent plastic pollution, according to Zha.
Polyethylene plastic, found in items such as bags, water bottles, and food packaging, is a major driver of plastic pollution. It can take over a thousand years to decompose naturally. Moreover, only a small portion of polyethylene produced worldwide is recycled, so the bacterial process could contribute to recycling this waste stream.
The team sought to understand how the bacteria could convert polyethylene’s carbon atoms into a biologically encoded silk protein, rather than simply eating the polymer as a food source.
The bacteria used can eat polyethylene naturally, but the researchers explored converting polyethylene’s carbon into a silk protein
Pseudomonas aeruginosa is the bacterium studied for its natural ability to consume polyethylene. The RPI team investigated how this microbe could engineer the carbon contained in polyethylene into a silk protein coded by its genes.
Surprisingly, the laboratory-grown bacteria produced silk protein at a yield comparable to some bacteria traditionally used in biomanufacturing.
The process of plastic recycling
The underlying biology has roots in ancient practices. “Essentially, bacteria ferment plastic. Fermentation is used to make and preserve foods such as cheese, bread, and wine, and in the biochemical industry to produce antibiotics, amino acids and organic acids”, the study authors explained.
To begin, the plastic is pre-digested to enable fermentation. As Zha notes, just as humans chew food to aid digestion, bacteria struggle to break the long polymer chains of polyethylene.
Zha and co-author Matteos Koffas collaborated with researchers at Argonne National Laboratory. They heated and pressurized the plastic to depolymerize it, creating a soft, waxy substance. A layer of this wax served as a food source for the bacteria in the growth vials, diverging from typical fermentation that relies on sugars as nutrients.
“Instead of feeding the bacteria with the cake, we feed it with the candles on the cake”, Zha explained.
As a hot plate slowly rotates the vial contents, the bacteria work. After 72 hours, the team separated the bacteria from the liquid, purified the silk protein, and freeze-dried it. The resulting material, resembling torn cotton balls, could potentially be spun or transformed into other useful forms.
An innovative approach
“What makes this process especially interesting is that it requires minimal energy and avoids toxic chemicals, unlike conventional plastic production”, Zha noted. “The world’s best chemists could not convert polyethylene into spider silk, but these bacteria can. We benefit from nature’s own designs.”
Before recycled spider silk products become widespread, researchers first aim to produce the silk protein more efficiently. “This study shows that these bacteria can convert plastic into spider silk. The next step is to explore how altering the bacteria or other aspects of the process could boost production”, Koffas said.
Reference work: DOI: 10.1186/s12934-023-02220-0