Anaerobic Transformation and Detoxification of Sulfamethoxazole by Sulfate-Reducing Enrichments and Desulfovibrio vulgaris.

Sulfamethoxazole (SMX) is a veterinary antibiotic that is not efficiently removed from wastewater by routine treatment and therefore can be detected widely in the environment. Here, we investigated whether microbial anaerobic transformation can contribute to the removal of SMX in constructed systems. We enriched SMX-transforming mixed cultures from sediment of a constructed wetland and from digester sludge of a wastewater treatment plant. Transformation of SMX was observed in both sulfate-reducing and methanogenic cultures, whereas nitrate-reducing cultures showed no SMX transformation. In sulfate-reducing cultures, up to 90% of an initial SMX concentration of 100-250 μM was removed within 6 weeks of incubation, and the experiments demonstrated that the transformation was microbially catalyzed. The transformation products in sulfate-reducing cultures were identified as the reduced and isomerized forms of the isoxazole SMX moiety. The transformation products did not spontaneously reoxidize to SMX after oxygen exposure, and their antibacterial activity was significantly decreased compared to SMX. Population analyses in sequential transfers of the sulfate-reducing cultures revealed a community shift toward the genus Desulfovibrio. We therefore tested a deposited strain of Desulfovibrio vulgaris Hildenborough for its capacity to transform SMX and observed the same transformation products and similar transformation rates as in the enrichment cultures. Our work suggests that an initial anaerobic step in wastewater treatment can reduce the concentration of SMX in effluents and could contribute to decreased SMX concentrations in the environment.