Selection and characterisation of diverse metaldehyde-degrading bacteria reveal a novel shared degradation pathway

Metaldehyde, a xenobiotic cyclic ether, is used as molluscicide of choice in agriculture and horticulture, but recently its detection in water above permitted levels has become a major cause of concern. Despite its use only since 1930s, microbial degradation of metaldehyde has arisen. Using a combination of enrichment culture and comparative genomics we isolated eight new metaldehyde-degrading strains and identified a highly-conserved cluster of three horizontally-transferred genes shared amongst five beta- and gamma-proteobacteria and absent from non-degrading closely related type strains. Chemical mutagenesis corroborated these genes are responsible for metaldehyde degradation. Other metaldehyde-degrading strains lack this pathway implying that multiple degradation mechanisms have evolved. We demonstrated accelerated biodegradation of metaldehyde in multiple soils, highlighting the importance of the biological component in the degradation process. Using a most probable number method we confirmed that the metaldehyde-degrading population is proliferating in response to metaldehyde, but no evident changes in the composition of the community as a whole were detected, indicating the process is governed by rare taxa. By bringing together traditional microbiological and modern molecular approaches we identified the first genetic determinants for the biological degradation of metaldehyde and paved the way for targeted bioremediation strategies for water treatment.