Reductive degradation of chloramphenicol by Geobacter metallireducens

Abstract

Geobacter metallireducens is known to be capable of removing nitroaromatic compounds via an oxidation mode. However, little attention has been paid to investigate the reductive removal of chlorinated nitroaromatic compounds by G. metallireducens. In this study, G. metallireducens was used to reduce chloramphenicol (CAP), a typical chlorinated nitroaromatic antibiotic. Cyclic voltammograms and chronoamperometry highlighted a higher peak current for CAP reduction by G. metallireducens compared to the control without bacteria. G. metallireducens efficiently reduced CAP (20 mg/L) with acetate as the sole electron donor, and the removal efficiency reached (97.6±4.9)% within 6 d. Aromatic amine (AMCl2), AMCl (dechlorinated AMCl2) and AM (dechlorinated AMCl) were identified as reduction products by liquid chromatography-mass spectrometry. However, the removal efficiency declined to (25.0±3.6)% when the CAP dosage increased to 80 mg/L. Transcriptomic analysis indicated the significant upregulation of genes related to electron transfer, such as pilus assembly protein gene (2.8 folds), NADH-quinone oxidoreductase subunit K2 gene (4.5 folds) and many c-type cytochrome genes such as cytochrome c biogenesis protein ResB (Gmet 2901, 4.6 folds), cytochrome c (Gmet 0335, 4.4 folds) and cytochrome c7 (Gmet 2902, 3.4 folds). Furthermore, a gene related to chlorinated contaminant removal (Gmet 1046, 5.4 folds) was also upregulated, possibly resulting in enhanced CAP reduction. This work deepened our knowledge of the bioremediation ability of G. metallireducens with respect to environmental contaminants and provided a potential strategy to treat antibiotics with electrochemically active bacteria.