Selection of polymer-degrading bacteria in a polylactic acid-trained mesophilic compost

Polylactic acid (PLA) is a bio-based polymer known to exhibit only thermophilic compostability. We recently developed a unique set of compost that can biodegrade PLA at mesophilic conditions, and it exhibited more than 10-fold higher PLA breakdown activity than control composts. Here, we investigated the potential selection of bioactivities related to PLA breakdown in our uniquely trained compost and surveyed the occurrences of those activities in a range of terrestrial and marine environments for comparison. We found that the fraction of PLA-hydrolyzing bacteria in our trained compost was not significantly increased compared to environmental samples. However, PLA-clearing strains isolated from the trained compost were twice as efficient in hydrolyzing PLA compared to those derived from environmental soil, indicating that our trained compost had undergone selection of strains with higher hydrolysis activity. This provides an important insight into PLA-related bioactivity in our trained compost, and it further suggests additional bioactivities that facilitate PLA breakdown most likely exist in our trained compost. Our findings support a multi-step model for mesophilic PLA composting in which several microbial-based bioactivities act synergistically to enhance PLA biodegradation. IMPORTANCE: We recently succeeded in developing a unique trained compost that can biodegrade the most common bioplastic, polylactic acid (PLA), by overcoming the high-temperature condition previously thought to be required for PLA biodegradation. In this present study, we investigated bioactivities in this trained compost of ours to understand how mesophilic PLA composting could be achieved, and we found that there was selection of more efficient PLA-hydrolyzing strains in the trained compost. Notably, PLA compostability in our trained compost is much higher than that in controls, and this cannot be fully explained by the increase in PLA hydrolysis activity alone, strongly suggesting that there are additional bioactivities in the trained compost that facilitate PLA breakdown. Therefore, we propose that mesophilic PLA biodegradation requires the collective action of several distinct microbial activities.