Plastic-Eating Bacteria in Farm Soil: *Bacillus* PE4 Breaks Down Polyethylene

Plastic mulch strips abandoned in agricultural soil after harvesting have long appeared to be a waste-clearance issue rather than a source of scientific discovery. The focus has been on removing them, not on the possibility that bacteria capable of consuming plastic could already be present beneath them.

Scientists in China’s Zhejiang province chose another route. Instead of searching landfill sites or marine sediments, they examined the earth directly below the films and identified microbes already carrying out this task.

Polyethylene: a persistent pollutant

Polyethylene is the resin used in these sheets and carrier bags, and it accounts for around a third of all plastic products made globally. Its extended carbon chains offer very few vulnerable sites for natural enzymes to attach to.

The material was designed specifically not to decay. Producers created it to last, but that same resilience makes it exceptionally difficult to remove after it enters the environment.

On farms that rely on plastic mulch, residual pieces accumulate annually. They gradually fragment into ever-smaller particles, ultimately becoming microplastics small enough to move through soil, water and crops.

A recent investigation of farmland surrounding greenhouses followed plastic debris as it travelled far beyond the fields where it originated. These fragments do not remain where they were first placed.

Examining farm soil in Zhejiang

Researchers at Hangzhou Normal University (HNU) asked whether microbes living alongside buried plastic had adapted to consume it. Bacteria respond quickly to constant pressure, and agricultural soils have provided that pressure for decades.

Biotechnologist Lu-E Shi and colleagues at HNU gathered soil from fields on China’s eastern Zhejiang coast, where mulch films have been used by farmers for many years. The researchers combined the samples with shredded polyethylene, providing no alternative food source.

Any microbes that endured would need to use the plastic as nourishment. Five bacterial strains survived, each from the genus Bacillus - common soil microbes recognised for decomposing resilient organic matter.

Bacillus PE4, a plastic-eating bacterium

The most notable of the plastic-eating bacteria was designated Bacillus PE4. Left with thin polyethylene films for 30 days, it reduced the plastic’s starting weight by almost 5%.

That figure may seem small, but without assistance the same film could remain in soil for centuries.

The researchers also supplied PE4 with powdered polyethylene, creating a direct comparison that had not previously been conducted so systematically.

Viewed under a high-powered microscope, the film’s surface revealed the effects. A surface that had initially been smooth and shiny was marked by pits and fissures where the bacterium had colonised it.

This deterioration was caused by bacteria alone, rather than by heat, light or chemical exposure.

Optimising the conditions

Having identified an effective strain, the team tested its limits. They assessed it across different pH values and initial bacterial populations, and also pre-treated the polyethylene with heat to make its surface softer.

The strongest results came from mildly alkaline conditions, an initial bacterial dose of 4%, and heat pre-treatment.

Combined, these adjustments increased degradation from around 5% over one month to approximately 28% in 100 days.

That improvement turns an interesting observation into a process with potential for further development. Rather than merely grazing on the plastic, the bacterium was actively breaking it down in conditions that a wastewater treatment plant or composting facility could realistically replicate.

How plastic-eating bacteria break down polyethylene

The chemical findings explained the improvement. As Bacillus PE4 acted on the plastic, two particular enzymes - of the sort evolved to break down durable carbon-based material in wood - increased sharply outside the bacterial cells.

Polyethylene’s resistant carbon backbone is chemically comparable with those natural materials. This resemblance may account for the ability of the same enzyme system to act on synthetic plastic.

Previous research involving related Bacillus strains had suggested this very mechanism: wood-degrading tools performing a second role on plastic. This study makes the evidence clearer, showing that higher enzyme activity coincided with greater film damage.

The next steps for Bacillus PE4

Until now, direct comparisons of the way one microbe processes different physical versions of plastic - a solid film and loose powder - have been limited. These findings address that shortfall.

The strain was also found in an unexpected setting. Earlier searches for plastic-eating microbes have concentrated on landfill sites, the digestive systems of insects that eat plastic, and deep-sea sediments. Everyday agricultural soil had not been considered a prime location for plastic-eating bacteria.

For practical applications, this difference matters. A bacterium able to work on a thick film may be suited to larger plastic waste, whereas one that acts on powder could be combined with shredding to deal with microplastics already present in farm soil.

Bacillus PE4 appears to be a prospect for both uses. However, the team does not present it as a complete answer: breaking down 28% in 100 days under optimised conditions is not a treatment ready to be applied across a field tomorrow.

Even so, the study offers the sector a new starter strain and a more precise understanding of which enzymes could be engineered in pursuit of faster ones. For an issue that has defied progress for decades, that is significant.