Harnessing economical biopolymer extrusion: the Bacillus clade as endotoxin-free platforms for next-generation bioprocesses

Biotechnological fermentation of polyhydroxyalkanoates (PHAs) from microbes is rooted in decelerating the reliance on synthetic plastics, one of the predominant challenges for the sustainable development goals (SDGs) of recent decades. The multifaceted inherent properties of these PHAs also exert wide spectrum applicability in numerous industrial, environmental, and healthcare sectors. However, conventional producers include gram-negative microbes with stringent nutrient requirements, low PHA productivities, and endotoxin-contaminated products thereby limiting large-scale production. We hereby critically review the inherent potential of developing non-pathogenic gram-positive Bacillus cereus clade as the chassis for PHA biosynthesis and growth-dependent (exponential) accumulation with high purity. Integration of these microbes as PHA producers in mainstream industries requires in-depth and precise knowledge that is provided within this review in coordination with (i) key operons/pathways, (ii) evolved regulatory mechanisms, (iii) toxigenicity evasion, (iv) carbon flux engineering, and (v) -omics-supported bioprocesses. Among them, the review reports newly updated Bacillus emend cereus members with class IV PhaC ‘synthase’ demonstrating superior properties such as broad substrate specificity, structurally unrelated waste carbon catalysis, diverse monomeric copolymerization and unique alcoholytic cleavage. Moreover, the obtained biopolymer naturally lacks pyrogenic contamination meaning that the end polymer is in compliance with the Food and Drug Administration (FDA). Accordingly, this can propel the industrial B. cereus clade PHAs in advanced biorefinery domains using second-generation (waste) feedstocks to promote a circular economy, reduce the carbon footprint and an increase in practical applications as both social and environmentally friendly polymers.