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Exploring the potential of earthworm gut bacteria for plastic degradation
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Researchers tested five species of earthworm gut bacteria against four common plastic types and found that only one bacterium — Streptomyces fulvissimus — showed meaningful ability to degrade polylactic acid (PLA) plastic, and only at 30°C with a small carbon food source, pointing to a narrow but real bioremediation potential.
The use of plastic mulch films in agriculture leads to the inevitable accumulation of plastic debris in soils. Here, we explored the potential of earthworm gut-inhabiting bacterial strains (Mycobacterium vanbaalenii (MV), Rhodococcus jostii (RJ), Streptomyces fulvissimus (SF), Bacillus simplex (BS), and Sporosarcina globispora (SG) to degrade plastic films (⌀ = 15 mm) made from commonly used polymers: low-density polyethylene film (LDPE-f), polylactic acid (PLA-f), polybutylene adipate terephthalate film (PBAT-f), and a commercial biodegradable mulch film, Bionov-B® (composed of Mater-Bi, a feedstock with PBAT, PLA and other chemical compounds). A 180-day experiment was conducted at room temperature (x̄ =19.4 °C) for different strain-plastic combinations under a low carbon media (0.1× tryptic soy broth). Results showed that the tested strain-plastic combinations did not facilitate the degradation of LDPE-f (treated with RJ and SF), PBAT-f (treated with BS and SG), and Bionov-B (treated with BS, MV, and SG). However, incubating PLA-f with SF triggered a reduction in the molecular weights and an increase in crystallinity. Therefore, we used PLA-f as model plastic to study the influence of temperature ("room temperature" & "30 °C"), carbon source ("carbon-free" & "low carbon supply"), and strain interactions ("single strains" & "strain mixtures") on PLA degradation. SF and SF + RJ treatments significantly fostered PLA degradation under 30 °C in a low-carbon media. PLA-f did not show any degradation in carbon-free media treatments. The competition between different strains in the same system likely hindered the performance of PLA-degrading strains. A positive correlation between the final pH of culture media and PLA-f weight loss was observed, which might reflect the pH-dependent hydrolysis mechanism of PLA. Our results situate SF and its co-culture with RJ strains as possible accelerators of PLA degradation in temperatures below PLA glass transition temperature (Tg). Further studies are needed to test the bioremediation feasibility in soils.
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Researchers isolated bacteria from earthworm guts that were able to degrade low-density polyethylene, demonstrating that intestinal microbes from soil invertebrates may play a role in plastic breakdown. The findings suggest that earthworm gut microbiomes are a reservoir of plastic-degrading bacteria with potential applications for bioremediation of LDPE-contaminated soils.
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Polylactic acid (PLA), a compostable bioplastic, was found to degrade more rapidly when it passes through earthworm guts due to microbial activity in the earthworm digestive system. This suggests that earthworms may play a role in accelerating the breakdown of biodegradable plastics in soil. However, if PLA degrades incompletely it could still generate microplastic fragments, so biodegradability claims need to be evaluated under realistic soil conditions.
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