Polylactic Acid (PLA)-Degrading Microorganisms and PLA Depolymerases

This paper summarizes topics on microorganisms able to degrade polylactic acid (PLA) and PLA depolymerases. Although the glass transition temperature for PLA is high (approximately 60 °C), PLA-degrading microbes can degrade solid PLA at far lower temperatures such as 30 or 37 °C. Such degraders are Actinomycetes belonging to family Nocardiaceae: PLA depolymerases were purified and cloned as serine proteases from genus Amycolatopsis. Thermophilic lipases were obtained from thermophilic Bacillus strains able to grow on PLA at 60 °C, although their contribution to degradation of PLA is skeptical at high temperature as PLA is easily hydrolyzable. Commercially available proteases and lipases are known to act as PLA depolymerases. We found that enantioselectivity of protease-type depolymerases is specific to poly(L-lactic acid), but that of lipase-type depolymerases is preferential to poly(D-lactic acid). Thus, proteases and lipases are categorized into two different classes of PLA depolymerases.