Anoxic microbial transformation of antibiotics by Nitratidesulfovibrio vulgaris

Introduction and objectives

Wastewater treatment plants do not fully remove antibiotic residuals such as sulfonamides and their human metabolites and hence discharge considerable amounts into surface water leading to detection even in groundwater. Bank filtration represents a well-established process to eliminate antibiotic residuals. Despite the essential role of redox zone sequences during bank filtration to naturally attenuate antibiotic residuals, the responsible microbial community governing their fate and fate of their transformation products (TPs) are not fully revealed. Particularly under sulfate reducing conditions sulfonamide antibiotics can be transformed, however knowledge about the molecular process, the microbial communities and the extent of antibiotics transformation is scarce.

The objective of the presented study was to investigate the microbial transformation of several classes of antibiotics and their human metabolites by the strain Nitratidesulfovibrio vulgaris. Additionally, we aimed to elucidate transformation products and pathways and link them to the anaerobic metabolism of N. vulgaris using different electron donors and acceptors.

Methods

We conducted batch experiments cultivating N. vulgaris coupling the oxidation of various electron donors to the dissimilatory reduction of different electron acceptors with given antibiotics at concentrations between 10 and 650 µM. Tandem mass spectrometry was employed to investigate the formation and persistence of TPs and perform shotgun proteomics. Additionally, spectral photometry and ion chromatography were used to monitor growth and catabolic activity of N. vulgaris.

Results

We observed the cometabolic transformation of the antibiotic sulfamethoxazole (SMX) and it´s human metabolite N-Acetyl-SMX at 10 to 650 µM under sulfate-reducing conditions using lactate or hydrogen as electron donors. We identified several TPs with shorter half-lives then SMX also suggesting different transformation mechanism, e.g. isoxazole ring cleavage. Comparative proteomics suggested house-keeping enzymes to transform both compounds. The antibiotics sulfamethazine, sulfadiazine and trimethoprim were not transformed.

Conclusion

The conducted batch experiments showed the capability of N. vulgaris to cometabolically transform several antibiotics and their human metabolites under anoxic conditions highlighting the important role of anoxic conditions for their natural attenuation.