Characterization of Soil Bacterial Isolates Capable of Degrading Biodegradable Plastic Mulch Films.

Plastic mulch films have agronomic benefits such as earlier harvest times, higher yields, reduced weeds, and improved moisture conservation. Polyethylene (PE) plastic is the most common feedstock, which poses considerable financial and environmental burdens associated with disposal after harvest. Biodegradable plastic mulch films (BDMs) are a sustainable alternative because BDMs are tilled into the soil and degraded by microbes. However, breakdown in soil can be unpredictable and the lack of research and laboratory models has precluded the understanding of the microbial degradation mechanisms. To solve this, microorganisms were extracted from soil and grown on minimal media enriched with squares of BDM films. Once pure cultures were obtained, 16S rRNA gene sequencing identified the isolates as Bacillus, Rhodococcus and two different Streptomyces sp. The isolates were tested for degradation of two BDMs (BioAgri® and an experimental PLA/PHA film) and conventional black polyethylene plastic. All strains were capable of degrading all three plastics to some extent: BioAgri® was the most readily degraded, with the Bacillus exhibiting greatest degradation rates. Next, to study their polylactic acid (PLA) degrading capabilities we incubated the strains in PLA-coated vials and quantified the production of L-lactate, its degradation by-product. Although L-lactate levels were not significant when grown with PLA only, we observed several fold changes in its production when glucose was added to the media. Additionally, a survey of other commonly used carbon sources indicated that the two strains had different preferences for simple carbon sources associated with PLA degradation. Taken to together, this indicates that different bacterial species are likely using different metabolic pathways and strategies in the degradation of biodegradable polymers. This work has identified and begun to characterize the degradation of BDMs by bacterial isolates, an important first step towards revealing the mechanisms of degradation in soil.