Characterising the Multiple-Plastic Degrading Strain of Bacillus subtilis GM_03 From the Galleria mellonella Microbiome

Plastic waste is a mounting global problem with over 400 million tons of plastic produced annually and over 50% ending up in landfill after its intended use. Two types of plastics are particularly problematic and are difficult to recycle: low-density polyethylene (LDPE) and polyurethane (PU). Fortuitously, nature may offer a potential solution; Galleria mellonella larvae can digest various plastics, including LDPE, which is believed to be driven by microbes in their gut microbiome. Although some studies have examined their gut microbiota on a metagenomic level, little is known about their ability to degrade plastics. Here, we isolated six bacterial strains from G. mellonella larvae feeding on LDPE. One of them, identified as Bacillus subtilis GM_03, has the capacity to break down commercial PU (Impranil), in addition to LDPE. This bacterium encodes a suite of genes required for plastic degradation. Directed evolution was used to enhance this strain's plastic degrading rate by over six-fold. Sequencing of the evolved culture revealed four genes, srfAB, fadD, appA and citS, associated with this increased PU degradation rate. This is the first time that B. subtilis isolated from G. mellonella larvae has been shown to be capable of degrading multiple types of plastics.