Plastic is notoriously hard to break down, but a study by researchers in Austria has shown that bacteria from a cow’s rumen—one of the four compartments of its stomach—can digest certain types of man-made polyester plastics, including poly(ethylene terephthalate), or PET. The scientists, headed by a team at the University of Vienna, had suspected that bacteria in the cow’s digestive system might be useful for PET degradation, given that the animals’ diets already contain natural plant polyesters.
“A huge microbial community lives in the rumen reticulum and is responsible for the digestion of food in the animals,” said research lead Doris Ribitsch, PhD, of the University of Natural Resources and Life Sciences in Vienna, “so we suspected that some biological activities could also be used for polyester hydrolysis.” Their newly reported studies found that each of the three plastics they tested was broken down by microorganisms in the cow’s stomach, with the results indicating that the collective microbial community may act synergistically to result in more effective plastic degradation. The team suggests that their results could feasibly point to the development of a sustainable way of reducing plastic litter.
Ribitsch and colleagues reported on their studies in Frontiers in Bioengineering and Biotechnology, in a paper titled, “Together is better: the rumen microbial community as biological toolbox for degradation of synthetic polyesters.”
Plastic waste represents a “huge environmental problem,” the authors wrote, with millions of tons of plastic waste accumulating on land and in the marine environment. Increasing public pressure and legislation is accelerating the drive to develop new strategies to reduce the problem. Over the last 20 years, the investigators continued, “the concept of bio-economy” has motivated the scientific community to focus on ways of producing bio-based and/or biodegradable polyesters, along with technologies for microbial or enzymatic recycling. Using highly specific enzymes could all for step-wise recovery of high valuable building blocks from blended plastics or mixed waste streams.
Previous studies by Ribitsch’s team, and by other researchers, have demonstrated the potential use of enzymes for the hydrolysis of PET, perhaps the most important synthetic polyester, which is used in a wide range of applications, including textiles and packaging. Polyester hydrolysis is a type of chemical reaction that results in decomposition.
Ruminants, such as cows, are herbivorous mammals that have evolved a large forestomach, or rumen in which the animals’ ingested food undergoes microbial degradation before “real animal digestion,” the authors noted. In adult cattle the rumen chamber may have a capacity of 50–100 L.
The rumen contents include the forage, sediment, liquids, and microbial cells, which collectively providing ecosystems that the investigators said can be defined as a “chemostat,” where different biochemical reactions take place.
Given that the diet of ruminants may contain natural plant polyesters (such as cutin from berries, for example), it’s feasible that the rumen may contain the enzymes that can hydrolyze other synthetic polyesters, such as PET. For their reported studies the team obtained obtained rumen liquid from a slaughterhouse in Austria. They then incubated the rumen liquid with both powder and film forms of three types of polyester plastic, to evaluate how effectively each plastic would break down in the rumen liquid.
The three plastics included PET, polybutylene adipate terephthalate (PBAT), which is a biodegradable plastic often used in compostable plastic bags, and polyethylene furanoate (PEF), which is a biobased material made from renewable resources.
The results of their studies showed that all three plastics could be broken down by the microorganisms from cow rumen, with the plastic powders breaking down more quickly than plastic film. Compared with similar research that has been carried out to investigate plastic breakdown by single microorganisms, Ribitsch and her colleagues found that the rumen liquid was more effective, which might indicate that the microbial community could have a synergistic advantage, such that its the combination of enzymes, rather than any one particular enzyme, is what makes the difference. Compared to published data for pure enzymes and/or supernatants of single microorganisms, the polyester hydrolyzing activity of rumen fluid was relatively high. Apparently, not only one type of enzymes is present in the rumen mixture, but rather synergistic action of different esterases, lipases, or cutinases may occur.”
The team separately used shotgun metagnomics technology to analyze DNA extracted from the rumen content, and these results indicated that the overall microbial community comprised 98% bacteria, 1% eurkaryotic organisms, 0.9% Archaea, and less than 0.1% viral entities. Interestingly, they pointed out, “Among the bacteria, fungi, and archaea identified in this study by microbial community analysis, several of the most abundant species have been described to produce enzymes potentially capable of polyester hydrolysis.”
While the team’s work was only carried out at a lab scale, Ribitsch pointed out that, “Due to the large amount of rumen that accumulates every day in slaughterhouses, upscaling would be easy to imagine.” However, she also cautions that such research can be cost-prohibitive, as the lab equipment is expensive, and such studies require pre-studies to examine microorganisms. The researchers concluded, “Despite the fact that rumen fluid could be a cheap source for polymer degrading enzymes, future studies should aim at identification and cultivation of the microbes and enzymes involved in synergistic hydrolysis of polyesters as well as on possible community changes during incubation with polyesters.”