Microbial teamwork enables efficient breakdown of phthalate plastic pollutants

Plastic trash has reached the world's most remote locations, from the bottom of the Mariana Trench to the summit of Everest. Hundreds of plastic-eating microbes that could help us clean up have been discovered over the past quarter of a century, but there is a long way to go before they can be put to work in natural environments: microbial digestion of plastic is still slow, requires high temperatures, and only proceeds efficiently in bioreactors. Moreover, most plastic-eating microbes discovered so far can only digest a single kind of plastic.

One solution would be to combine different microbes to tackle plastic pollution as a team. This allows them to share tasks, compensate for each other's weaknesses, and continue working even when environmental conditions change. Now, scientists in Germany have discovered such a synergistic 'consortium' of plastic-eating bacteria, which can eat phthalate esters (PAEs) – plasticizers which are often found in building materials, food packages, and personal care products, but have been implicated in hormonal, metabolic, and developmental disorders and some cancers. The results are published in Frontiers in Microbiology.

"Here we show the degradation of various phthalate esters (PAEs) through the cooperative activity of several bacterial strains," said corresponding author Dr Christian Eberlein, a postdoctoral researcher at the Helmholtz Centre for Environmental Research in Leipzig. Eberlein and his colleagues are participants in the Helmholtz Sustainability Challenge project FINEST, which aims to engineer new solutions for a sustainable circular economy.

Strength through diversity

Eberlein and colleagues knew of a promising place to look for new plastic-eating microbes: in their own laboratory, eking out a living as a biofilm on the polyurethane tubing of a bioreactor. They scraped off a sample and incubated it in a growth medium, using the PAE diethyl phthalate (DEP) as the carbon and energy source. They focused on DEP because it is the typical model compound used in experiments with phthalate ester plasticizers. After serial transfers between cultures, they ended up with a stable community that could grow in DEP concentrations of up to 888 milligrams per liter. At 30 °C, the consortium needed 24 hours to completely devour the DEP.

DNA sequencing showed that three species of bacteria made up the consortium: one species each from the Pseudomonas putida and Pseudomonas fluorescens groups plus an unknown Microbacterium species.

The bacteria were unable to digest PAEs on their own, proving that they must work as a cooperative. Further tests showed that this synergistic superpower is due to so-called 'cross-feeding' where one microbe releases metabolic byproducts that its partner takes up as nutrients – thus sharing resources to create stable, diverse communities. Cross-feeding is a fundamental feature of microbial communities in nature, but had never before been demonstrated in plastic-eating bacteria. In this case, key intermediary products turned out to be PAEs themselves: monoethyl phthalate and phthalate. Proteomic analysis showed that the enzymes needed to break down these compounds are new to science.

Importantly, the consortium is metabolically versatile: besides DEP, it was able to digest dimethyl phthalate, dipropyl phthalate, and dibutyl phthalate – all common PAEs.

"This broad substrate range enhances the potential value of the consortium for biotechnological and environmental applications, as it can degrade multiple PAEs commonly found as plasticizers in contaminated environments," wrote the authors.

Recent evolution driven by the Plastic Age

How did this remarkable capacity to digest PAEs evolve?

"The initial reactions rely on pre-existing enzymes that originally evolved to break down natural molecules that contain ester bonds. Since then, persistent contamination with PAEs in nature has presumably created a strong evolutionary pressure, forcing microbes to adapt and develop more specialized enzymes that can break down PAEs much more efficiently," speculated Eberlein.

The consortium can't yet handle other types of plastics than PAEs. For example, polyethylene and polypropylene contain highly resistant non-ester bonds, which are inaccessible to natural enzymes.

The next step will be to test our new consortium in actual wastewater samples containing microplastics, to assess its ability to remove PAEs. Introducing these bacteria into polluted natural environments, a process known as bioaugmentation, could potentially help reduce PAE contamination in real-world settings."

Dr. Hermann Heipieper, senior scientist at the Helmholtz Centre and study's senior author