Physicochemical and Structural Evidence that Bacillus cereus Isolated from the Gut of Waxworms (Galleria mellonella Larvae) Biodegrades Polypropylene Efficiently In Vitro

Biodegradation of plastic waste using microorganisms has been proposed as one of the solutions to the increasing worldwide plastic waste. Polypropylene (PP) is the second most used plastic used in various industries, and it has been widely used in the production of personal protective equipment such as masks due to the COVID-19 pandemic. Therefore, biodegradation of PP becomes very important. Here, we present results on the physicochemical and structural studies of PP biodegradation by Bacillus cereus isolated from the gut of the waxworms, Galleria mellonella larvae. We also studied the biodegradability of PP by the gut microbiota compared with Bacillus cereus. We analyzed the microbial degradation of the PP surface using scanning electron microscopy and energy - dispersive X-ray spectroscopy and confirmed that the physical and chemical changes were caused by Bacillus cereus and the gut microbiota. The chemical structural changes were further investigated using X-ray photoelectron microscopy and Fourier - transform - infrared spectroscopy, and it was confirmed that the oxidation of the PP surface proceeded with the formation of carbonyl groups (C=O), ester groups (C–O), and hydroxyl groups (–OH) by Bacillus cereus. Additionally, the gut microbiota composed of diverse microbial species showed equal oxidation of PP compared to Bacillus cereus. More importantly, high temperature gel permeation chromatography (HT-GPC) analysis showed that Bacillus cereus exhibited quantitatively a higher biodegradability of PP compared to the gut microbiota. Our results suggest that Bacillus cereus possesses a complete set of enzymes required to initiate the oxidation of the carbon chain of PP and will be used to discover new enzymes and genes that are involved in degrading PP.