Biodegradation of vulcanized rubber by a gut bacterium from plastic-eating mealworms

Exploring the toxicology, socio-ecological impacts and biodegradation of microplastics in Africa: Potentials for resource conservation

Achieving upcycling and circularity in the microplastic economy predominantly depends on collecting and sorting plastic waste from the source to the end-user for resource conservation. Microplastics, whether from packaging or non-packaging materials, pose a significant environmental challenge as they are often not prioritized for collection or recycling initiatives. The presence of additives impedes the quality of plastic recyclates and the persistence of microplastics as shredded resultants remain a threat to the aquatic and terrestrial ecosystem and its biodiversity. Despite the increasing global research on microplastics, the success of plastic and microplastic waste management in Africa is yet to be fully attained. Considering the improper disposal, limited recycling and upcycling intervention, lack of policy, and strict laws against plastic waste management defaulters, the ecosystems in Africa remain immensely impacted by several socio-ecological factors leading to the loss of aquatic organisms through reducing fertility and increasing stress. As a ripple consequence, the disruption of economic activities, toxic effects on animal/human health, and climate crisis are among their impact. This review therefore provides comprehensive detail of microplastic production and challenges in Africa, the toxicology concerns, socio-ecological issues associated with microplastic waste management, and insight into approaches to mitigate plastic pollution through recycling, upcycling, bioprocessing and their biodegradation with social insects and microorganisms which may form the basis for adoption by policymakers and researchers, thereby minimizing the consequences of plastic pollution in Africa.

Discovery of two novel cutinases from a gut yeast of plastic-eating mealworm for polyester depolymerization

Identification of novel plastic-degrading enzymes is crucial for developing enzymatic degradation and recycling strategies for plastic waste. Here, we report the discovery of two novel cutinases, SiCut1 and SiCut2, from a yeast strain Sakaguchia sp. BIT-D3 was isolated from the gut of plastic-eating mealworms. Their amino acid sequences share less than 25% identity with all previously described cutinases and reveal a conserved S-D-H catalytic triad with a unique GYSKG motif. Their recombinant proteins were successfully overexpressed in Pichia pastoris. The pH range for both enzymes was 4.0 to 11.0 and the temperature range for SiCut1 and SiCut2 was 10°C to 50°C and 10°C to 70°C, respectively. Both enzymes showed strong activity against apple cutin and short-chain fatty acid esters of p-nitrophenol and glycerol, substantiating their classification as true cutinases. SiCut1 and SiCut2 have been demonstrated to exhibit efficient degradation of polycaprolactone (PCL) film, polybutylene succinate (PBS) film, and polyester-polyurethane (PUR) foam. Molecular docking and molecular dynamics simulations were used to elucidate the underlying mechanisms of the observed catalytic activity and thermal stability. This study shows that SiCut1 and SiCut2 are novel yeast-derived cutinases with the potential for depolymerization and recycling of plastic waste.IMPORTANCEThe identification of novel plastic-degrading enzymes is critical in addressing the pervasive problem of plastic pollution. This study presents two unique cutinases, SiCut1 and SiCut2, derived from the yeast Sakaguchia sp. BIT-D3 isolated from the gut of plastic-feeding mealworms. Despite sharing less than 25% sequence identity with known cutinases, both enzymes exhibit remarkable degradation capabilities against various polyester plastics, including polycaprolactone (PCL) film, polybutylene succinate (PBS) film, and polyester-polyurethane (PUR) foam. Our results elucidate the catalytic mechanisms of SiCut1 and SiCut2 and provide insights into their potential applications in enzymatic degradation and recycling strategies. By harnessing the gut microbiota of plastic-degrading organisms, this research lays the foundation for innovative enzyme-based solutions to reduce plastic waste and promote sustainable practices in waste management.

Size- and duration-dependent toxicity of heavy vehicle tire wear particles in zebrafish

Tire wear particles (TWPs), as a pervasive environmental pollutant, pose significant risks to aquatic ecosystems. This study investigates the effects of small (HS) and large (HL) TWPs produced by heavy vehicles on zebrafish, focusing on physiological, microbial, and transcriptomic levels, as well as their intergenerational consequences, under short-term (15 days) and long-term (90 days) exposure. Short-term exposure to small particles (HS15) significantly reduced body width and triggered widespread oxidative stress, while long-term exposure to large particles (HL90) increased gut weight and decreased gill weight, reflecting respiratory and digestive disruptions. Tissue-level analyses revealed that smaller particles accumulated more readily in internal organs, whereas larger particles caused localized physiological stress. Gut microbiota profiling indicated a marked decline in microbial diversity, compositional shifts, and network simplification, with HL15 enriched in Acinetobacter and xenobiotic metabolism pathways, and HS15 exhibiting Proteobacteria-dominated dysbiosis and enrichment of LPS biosynthesis genes. Liver transcriptomics revealed group-specific responses: HL15 exposure activated innate immunity via the NOD-MAPK axis, while HS15 induced atypical PI3K-NF-κB signaling, potentially linked to microbial LPS. Notably, all TWP-exposed groups showed enrichment of the herpes simplex virus 1 (HSV-1) infection pathway, suggesting a conserved antiviral-like host response. Transgenerational effects were evidenced by impaired growth and significant downregulation of GH/IGF signaling and upregulation of apoptotic genes in offspring, despite only subtle transcriptomic changes in long-term exposed parents. These findings underscore the importance of particle size, exposure duration, and microbiota-gut-liver axis interactions in mediating TWP toxicity and highlight potential transgenerational risks associated with environmental microplastic exposure.

Using Insect Larvae and Their Microbiota for Plastic Degradation

Plastic pollution is one of the biggest current global threats to the environment given that petroleum-based plastic is recalcitrant and can stay in the environment for decades, even centuries, depending on the specific plastic type. Since less than 10% of all plastic made is recycled, and the other solutions (such as incineration or landfill storage) are pollutant methods, new, environmentally friendly solutions are needed. In this regard, the latest biotechnological discovery on this topic is the capability of insect larvae to use plastic polymers as carbon feedstock. This present review describes the most relevant information on the insect larvae capable of degrading plastic, mainly Galleria mellonella (Fabricius, 1798), Tenebrio molitor (Linnaeus, 1758), and Zophobas atratus (Fabricius, 1776), and also adds new information about other less commonly studied "plastivore" insects such as termites. This review covers the literature from the very first work describing plastic degradation by larvae published in 2014 all the way to the very latest research available (till June 2024), focusing on the identification of a wide variety of plastic-degrading microorganisms isolated from larvae guts and on the understanding of the potential molecular mechanisms present for degradation to take place. It also describes the latest discoveries, which include the identification of novel enzymes from waxworm saliva.

Discovery of a polyurethane-degrading enzyme from the gut bacterium of plastic-eating mealworms

Although numerous polyurethane (PU)-degrading enzymes were identified from a diverse array of microorganisms in soil or compost, it is intriguing to investigate whether novel PU-degrading enzymes can be discovered in other biological environments. This study reports the discovery of an enzyme (MTL) for PU plastic degradation from the bacterial strain Mixta tenebrionis BIT-26, isolated from the gut of plastic-eating mealworms. MTL shows significant degradation activity towards three commercial PU substrates, including Impranil®DLN-SD, thermoplastic films (PEGA-HDI), and thermoset foams (PEGA-TDI), by cleaving the ester bonds in the polyester polyol moieties. The structure, molecular docking, and site-directed mutagenesis analyses elucidate the substrate binding model. A combination of structure-based comparison and mutational studies reveals the underlying architecture of the enzyme's specificity. These findings provide a fresh perspective into understanding plastic metabolism in the gut of plastic-eating insects and a prospective path for developing a biodegradation technique for plastic waste disposal.

Bispillar[5]arene-Based Slide-Ring Polyrotaxanation Enables Enhanced Toughness, Recyclability, Impact, and Puncture Resistance of Polyisoprene Elastomers

A series of slide-ring polyrotaxanes (SRPs) have been constructed by the solvent-free blending of a ditopic pillar[5]arene (DP5A) and polyisoprene (PIP) after thermal annealing. Solid-state 13C NMR experiments supported the fact that the pillar[5]arene rings of DP5A were threaded by PIP chains to afford physically interlocked networks. Tensile tests revealed that 1% of DP5A can improve the elongation at break from 50 to 239%, the tensile modulus from 2.1 to 3.9 MPa, and the toughness from 0.35 to 4.5 MJ/m3. Impact and puncture resistance experiments show that the DP5A-doped materials exhibit remarkable enhancement of protective and impalement-resistant performance. The samples can be also recycled repeatedly due to their physical crosslinking nature. The important stress delocalization effects have been attributed to the pulley effect of DP5A in the SRP materials, which represents a supramolecular approach for improving the performance of PIP elastomers.

Current paradigms and future challenges in harnessing gut bacterial symbionts of insects for biodegradation of plastic wastes

The ubiquitous incorporation of plastics into daily life, coupled with inefficient recycling practices, has resulted in the accumulation of millions of metric tons of plastic waste, that poses a serious threat to the Earth's sustainability. Plastic pollution, a global problem, disrupts the ecological balance and endangers various life forms. Efforts to combat plastic pollution are underway, with a promising avenue being biological degradation facilitated by certain insects and their symbiotic gut microorganisms, particularly bacteria. This review consolidates existing knowledge on plastic degradation by insects and their influence on gut microbiota. Additionally, it delves into the potential mechanisms employed by insects in symbiosis with gut bacteria, exploring the bioconversion of waste plastics into value-added biodegradable polymers through mineralization. These insights hold significant promise for the bio-upcycling of plastic waste, opening new horizons for future biomanufacturing of high-value chemicals from plastic-derived compounds. Finally, we weigh the pros and cons of future research endeavors related to the bioprospection of plastic-degrading bacteria from underexplored insect species. We also underscore the importance of bioengineering depolymerases with novel characteristics, aiming for their application in the remediation and valorization of waste plastics.

Study of sequential abiotic and biotic degradation of styrene butadiene rubber

Styrene butadiene rubber is one of the main constituents of tire tread. During tire life, the tread material undergoes different stresses that impact its structure and chemical composition. Wear particles are then released into the environment as weathered material. To understand their fate, it is important to start with a better characterization of abiotic and biotic degradation of the elastomer material. A multi-disciplinary approach was implemented to study the photo- and thermo- degradation of non-vulcanized SBR films containing 15 w% styrene as well as their potential biodegradation by Rhodoccocus ruber and Gordonia polyisoprenivorans bacterial strains. Each ageing process leads to crosslinking reactions, much surface oxidation of the films and the production of hundreds of short chain compounds. These degradation products present a high level of unsaturation and oxidation and can be released into water to become potential substrates for microorganisms. Both strains were able to degrade from 0.2 to 1.2 % (% ThOD) of the aged SBR film after 30-day incubation while no biodegradation was observed on the pristine material. A 25-75 % decrease in the signal intensity of water extractable compounds was observed, suggesting that biomass production was linked to the consumption of low-molecular-weight degradation products. These results evidence the positive impact of abiotic degradation on the biodegradation process of styrene butadiene rubber.

Biodegradation of polystyrene nanoplastics by Achromobacter xylosoxidans M9 offers a mealworm gut-derived solution for plastic pollution

Nanoplastics pose significant environmental problems due to their high mobility and increased toxicity. These particles can cause infertility and inflammation in aquatic organisms, disrupt microbial signaling and act as pollutants carrier. Despite extensive studies on their harmful impact on living organisms, the microbial degradation of nanoplastics is still under research. This study investigated the degradation of nanoplastics by isolating bacteria from the gut microbiome of Tenebrio molitor larvae fed various plastic diets. Five bacterial strains capable of degrading polystyrene were identified, with Achromobacter xylosoxidans M9 showing significant nanoplastic degradation abilities. Within 6 days, this strain reduced nanoplastic particle size by 92.3%, as confirmed by SEM and TEM analyses, and altered the chemical composition of the nanoplastics, indicating a potential for enhanced bioremediation strategies. The strain also caused a 7% weight loss in polystyrene film over 30 days, demonstrating its efficiency in degrading nanoplastics faster than polystyrene film. These findings might enhance plastic bioremediation strategies.

Investigation of potential rubber-degrading bacteria and genes involved

COVID-19 pandemic has generated high demand for natural rubber gloves (NR) leading to crucial issues of rubber waste and waste management such as burning, dumping, stockpiling, discarding waste in landfills. Hence, rubber biodegradation by microorganisms is an alternative solution to the problem. The biodegradation method is environmentally friendly but normally extremely slow. Numerous microorganisms can degrade NR as a source of carbon and energy. In this study, Rhodococcus pyridinivorans KU1 was isolated from the consortium CK from previous study. The 40% rubber weight loss was detected after incubated for 2 months. The bacterial colonization and cavities on the surface of rubber were identified using a scanning electron microscope (SEM). The result demonstrated the critical degradation of the rubber surface, indicating that bacteria can degrade rubber and use it as their sole carbon source. The result of whole-genome sequencing (WGS) revealed a gene that is 99.9% identical to lcp which is responsible for poly (cis-1,4-isoprene) degradation. The results from Meta16S rRNA sequencing showed that the microbial communities were slightly shifted during the 2-month degradation, depending on the presence of monomers or oligomers appeared during the degradation process. The majority of species were soil bacteria such as phylum Proteobacteria, Actinobacteria, and Firmicutes. Members of Pseudoxanthomonas seemed to be the dominant degraders throughout the degradation.

Assembly strategies for rubber-degrading microbial consortia based on omics tools

Numerous microorganisms, including bacteria and fungus, have been identified as capable of degrading rubber. Rubber biodegradation is still understudied due to its high stability and the lack of well-defined pathways and efficient enzymes involved in microorganism metabolism. However, rubber products manufacture and usage cause substantial environmental issues, and present physical-chemical methods involve dangerous chemical solvents, massive energy, and trash with health hazards. Eco-friendly solutions are required in this context, and biotechnological rubber treatment offers considerable promise. The structural and functional enzymes involved in poly (cis-1,4-isoprene) rubber and their cleavage mechanisms have been extensively studied. Similarly, novel bacterial strains capable of degrading polymers have been investigated. In contrast, relatively few studies have been conducted to establish natural rubber (NR) degrading bacterial consortia based on metagenomics, considering process optimization, cost effective approaches and larger scale experiments seeking practical and realistic applications. In light of the obstacles encountered during the constructing NR-degrading consortia, this study proposes the utilization of multi-omics tools to discern the underlying mechanisms and metabolites of rubber degradation, as well as associated enzymes and effective synthesized microbial consortia. In addition, the utilization of omics tool-based methods is suggested as a primary research direction for the development of synthesized microbial consortia in the future.

Acinetobacter entericus sp. nov., isolated from the gut of plastic-eating insect larvae Zophobas atratus

A Gram-negative, non-motile and rod-shaped strain, BIT-DXN8T, was isolated from the gut of plastic-eating insect larvae Zophobas atratus. The taxonomic position of this new isolate was examined by using a polyphasic approach. A preliminary analysis based on the 16S rRNA gene sequence (1411 bp) indicated that the most similar strain to BIT-DXN8T was Acinetobacter bouvetii DSM 14964T (98.5%), followed by Acinetobacter haemolyticus CIP 64.3T (98.2%) and Acinetobacter pullicarnis S23T (98.2%). The results of phylogenetic analyses, based on the 16S rRNA gene, concatenated sequences of five housekeeping genes (fusA, gyrB, recA, rplB and rpoB) and genome sequences, placed strain BIT-DXN8T in a separate lineage among the genus Acinetobacter of the family Moraxellaceae. The average nucleotide identity and digital DNA-DNA hybridization values of the strain when compared to all other species within the genus Acinetobacter were below 96 and 70 %, respectively. The physiological and biochemical tests confirm the affiliation of strain BIT-DXN8T to the present species within the genus Acinetobacter, but with some specific phenotypic differences. Therefore, strain BIT-DXN8T is considered to represent a novel species, for which the name Acinetobacter entericus sp. nov. is proposed. The type strain is BIT-DXN8T (=CCTCC AB 2022117T=KCTC 92696T).