Isolation and role of polylactic acid-degrading bacteria on degrading enzymes productions and PLA biodegradability at mesophilic conditions

Enumeration and isolation of polylactic acid (PLA)-degrading bacteria from soils and wastewater sludge were performed on emulsified PLA agar. Two isolates of potent PLA-degrading bacteria, designated as CH1 and WS3, were selected and identified as Stenotrophomonas pavanii and Pseudomonas geniculata, respectively. PLA was presented as a substrate to stimulate production of protease and PLA-degrading enzyme by S. pavanii CH1 and P. geniculata WS3. The optimal pH values for both protease and PLA-degrading enzyme production by S. pavanii CH1 and P. geniculata WS3 were 7.5 and 8.0, respectively. The optimal gelatin concentrations for stimulating protease production in S. pavanii CH1 and P. geniculata WS3 were 0.3% (w/v), while those for PLA-degrading enzyme production in S. pavanii CH1 and P. geniculata WS3 were 0.1 and 0.3% (w/v), respectively. In addition, P. geniculata WS3 had a higher percentage of PLA film-weight loss than that of S. pavanii CH1, corresponding with reduced molecular weight of PLA. A significant increase of lactic acid content in culture broth was directly correlated with the increasing percentage of PLA film-weight loss. Our results clearly demonstrated that P. geniculata WS3, a novel isolated bacterium, has played a substantial role in PLA biodegradation by producing PLA-degrading enzyme and adhering on PLA surface.