Cu²⁺ Improves Sulfamerazine Biodegradation by Sakaguchia cladiensis A5: Physiological Responses and Proteomic...

Sulfamerazine (SM1) and copper (Cu²⁺) are common co-contaminants in livestock and poultry farming environments, where they may threaten ecological safety and disrupt microbial communities. Microbial physiological and metabolic regulation governs pollutant degradation and transformation. However, research on the response and regulatory mechanisms of microbial cells to antibiotic-heavy metal co-stress remains limited, hindering the practical use of microbial remediation technologies. Building upon our previous transcriptomic profiling of transcriptional responses, the current study further extended the scope of investigation to cellular physiological responses, protein-level regulatory mechanisms, and the transformation process of SM1. Herein, Sakaguchia cladiensis A5 exhibited improved SM1 degradation in the presence of 10 mg·L^-1 Cu²⁺ with 1 mg·L^-1 SM1. A maximum removal efficiency of 54.1% was attained within 5 d, in contrast to <10% observed in the SM1-single system (1 mg·L^-1 SM1). Flow cytometry detected alterations in membrane integrity, mitochondrial membrane potential, oxidative-response fluorescence signals, and esterase activity upon SM1-Cu²⁺ co-exposure. Proteomic analysis identified pathway-scale reprogramming of membrane transport, hydrolase-/oxidoreductase-related functions, oxidative phosphorylation, fatty acid metabolism, and stress-response pathways. Fifteen tentative intermediates supported the proposal of four putative SM1 biodegradation pathways involving pyrimidine-ring cleavage, amino hydroxylation, potential SO₂ extrusion and Smiles-type rearrangement, and sulfonyl S-N bond cleavage followed by oxidative ring opening. Collectively, these findings offer novel mechanistic insights into microbial adaptation to antibiotic-metal co-stress, and underpin studies on physiological and molecular responses coupled to pollutant transformation.