Molecular insights into the high-efficiency catabolism of chloramphenicol for soil bioaugmentation by Nocardia testacea CS1

Screening microorganisms capable of efficiently degrading chloramphenicol (CAP) is critical for remediating CAP-contaminated soils. However, few bacterial strains capable of catabolizing CAP have been identified as suitable for soil application, and the metabolic and genetic mechanisms of CAP degradation remain poorly understood. Herein, Nocardia testacea CS1 was isolated for its ability to utilize CAP as the sole carbon source. CS1 exhibited substantial CAP mineralization and broad environmental tolerance across temperatures of 15–50 °C and pH values of 4–11. Integrated analyses of intermediate products, gene clusters, and transcriptomes revealed a proposed CAP catabolic pathway. The chloramphenicol degrading ( chd ) gene cluster was proposed to mediate sequential CAP transformation, including initial oxidation (ChdB), amide hydrolysis (ChdD), β-cleavage (ChdC), and transcriptional regulation (ChdR). Downstream nitroaromatic intermediates were further reduced by PnbAB and assimilated into central metabolism. Bioaugmentation experiments demonstrated effective CAP removal in farmland soils, supported by successful colonization and biofilm formation of CS1. Sustained CAP degradation was accompanied by improved soil ecological health. The degradation capabilities of CS1 extend to thiamphenicol and selected para -substituted nitroaromatic pollutants, while no activity was observed toward florfenicol or ortho / meta -substituted nitrobenzoates, indicating structural selectivity. This study demonstrates substantial CAP mineralization by a soil-derived Nocardia testacea strain and identifies a chd gene cluster associated with CAP catabolism, providing mechanistic insights for the bioremediation of CAP-contaminated soils.