Chloramphenicol biodegradation by Citrobacter freundii CT2: Degradation pathways, toxicity assessment, and genomic analysis

Chloramphenicol (CAP), a widely used antibiotic, persists in the environment and poses ecological risks. Despite the growing understanding of these degradation pathways of CAP, the secondary metabolic routes and their impacts on the environment remain poorly understood. Here, we aimed to characterize the CAP-biodegradation capability of a novel bacterial strain, evaluate the biodegradation kinetics of CAP under different nutritional and environmental conditions, and assess the toxicity of CAP and its degradation products. Citrobacter freundii CT2 isolated from fermented silkworm excrement was found to degrade CAP with an efficiency of 96.29% within 24 h. CAP is the sole source of carbon, nitrogen, and energy for this bacterium. The degradation process fitted well with the first-order kinetics model. Ten potential biotransformation products were identified using ultraperformance liquid chromatography–high-resolution mass spectrometry (UPLC-HRMS) and high-performance liquid chromatography (HPLC), and five distinct metabolic pathways (acetylation, propionylation, butyrylation, dehydration, and oxidation) were predicted. A toxicity assessment of the biotransformation products, conducted using biological indicators, showed that the final metabolites were non-toxic to the environment, confirming that the biodegradation process resulted in detoxification. Bio-enhancement studies further demonstrated that strain CT2 effectively degraded CAP in wastewater. Genomic analysis showed that genes related to acetylation, propionylation, nitro reductase and dehydrogenation in strain CT2 might be involved in the biodegradation of CAP. These results offer new insights into the diversity of CAP-degrading pathways and highlight the utility of strain CT2 as a potential bioremediation agent for CAP-contaminated environments.