Good news for sustainable textiles of the future was announced in a recent Applied and Environmental Microbiology report that described the discovery of microbial polyesterase genes. The study described the discovery and characterization of new enzymes from microbial communities associated with moss. These enzymes might some day be used to recycle polyesters, commonly used in textiles, into their monomeric building blocks.
The fast-fashion movement results in a lot of discarded clothing when last year’s fashions are no longer trendy. Many of the donated clothes given away by well-meaning individuals end up in landfills, where the poor oxygenation slows biodegradation of most materials. Discovery of polyester-degrading enzymes opens up many possible applications, from possible "clothing compost" to recovery of monomeric compounds to be reused rather than discarded.
Synthetic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are extremely resistant to microbial degradation. These polymers are commonly found in clothing and are also increasingly part of the plastic debris polluting our waters. First author Christina Müller, working with senior author Gabriele Berg and a team of scientists, set out to search for enzymes that could break the ester linkages of these polymeric compounds.
The team searched through the metagenome of the microbial community associated with Sphagnum magellanicum, a bog moss whose cell wall contains polysaccharides similar to those found in higher plants and which might require similar degradation as the synthetic polymers that compose plastics. Their genomic approach circumvented the need to culture individual species, and instead allowed the researchers to screen the function of the collective genes found within the microbial community.
Schematic of screen for polyester-degrading microbial genes Source.
The screen focused on identifying polyester-hydrolyzing activities from bacteria containing one of the 90,000 fosmid clones expressing bog moss-associated genes. The search began by screening these clones for polyester-hydrolyzing activity, identified by hydrolytic halos when grown on tributryin agar plates (see figure, right). The second step screened 83 halo-positive clones for additional p-nitrophenyl butyrate hydrolysis activity. Finally, the 11 most active clones were screened for degradation of two synthetic copolyesters. Six finalist clones showing high activity in all screen assays were then further characterized.
The six final esterases varied in gene size and the presence of a secretion signal, but all showed preferential activity toward polymers with short or medium chain length (maximum C10). Each of the six showed slightly different substrate preferences, confirming the identification of six independent esterases. Two esterases, EstB3 and EstC7, were purified from cell lysates and their kinetics measured, finding an optimal temperature for enzyme-mediated catalysis of 47.7°C for EstB3 and 50°C for EstC7, with EstB3 showing a broader pH range between the two. Different hydrolysis profiles may facilitate different future applications as the enzymes are developed for industrial purposes.
Given the large number of polymer-associated microbial communities, future discoveries may reveal still more enzymes to degrade polyesters and other synthetic materials. The discoveries here were made entirely based on useful sequences, but identifying the original genetic source may inform scientists more about optimal enzymatic conditions or partnering enzymes that work in tandem.
Credit: Textile image