Numerous microorganisms accumulate polyesters classified as polyhydroxyalkanoates (PHAs) as carbon and energy storage material when the growth condition is unfavorable in the presence of excess carbon source. Natural PHAs typically consist of various (R)-hydroxycarboxylic acids, and exhibit different material properties depending on the monomer composition. Such diversity comes from different metabolic pathways operating in the cell, and thus generating different monomers. Even more diverse PHAs can be produced by metabolically engineered microorganisms, which leads to the biosynthesis of non-natural polyesters containing lactate as a monomer. In order to make PHAs as useful polymers in our daily life, their production cost should be significantly lowered and material properties should be compatible with those produced by petrochemical industries. Metabolic engineering can address these issues by developing microbial strains capable of producing PHAs of desired material properties with high productivity and yield from inexpensive carbon sources. This commentary aims at peeking into the future of PHAs, focusing on the possible metabolic engineering strategies to be taken to achieve these goals.

Synthetic biology has recently been at the center of the world's attention as a new scientific and engineering discipline. It allows us to design and construct finely controllable metabolic and regulatory pathways, circuits, and networks, as well as create new enzymes, pathways, and even whole cells. With this great power of synthetic biology, we can develop new organisms that can efficiently produce new drugs to benefit human healthcare and superperforming microorganisms capable of producing chemicals, fuels, and materials from renewable biomass, without the use of fossil oil. Based on several successful examples reported, this commentary aims at peeking into the potential of synthetic biology.

Production of biodegradable polymer polyhydroxyalkanoates (PHAs) from industrial wastes exhibits several advantages such as recycle of waste and the production of high valuable products. To this end, this study aimed at isolating from the sewage treatment plant a PHA producing bacterium capable of utilizing wastes generated from biodiesel and food industries. A Bacillus cereus strain capable of producing poly(3-hydroxybutyrate) [P(3HB)] was isolated, which was followed by confirmation of P(3HB) accumulation by gaschromatographic analyses. Then, the effects of nutrient limitation on P(3HB) production by B. cereus was first examined. Cells cultured in a minimal medium under the limitation of nitrogen, potassium and sulfur suggested that nitrogen limitation allows the highest P(3HB) accumulation. Next, production of P(3HB) was examined from both waste of biodiesel production (crude glycerol) and waste from food industry (spent coffee grounds). Cells cultured in nitrogen-limited minimal medium supplemented crude glycerol and waste spent coffee grounds extract accumulated P(3HB) to the contents of 2.4% and 1.0% of DCW. This is the first report demonstrating the capability of B. cereus to produce P(3HB) from waste raw materials such as crude glycerol and spent coffee grounds.