Abstract
Organic micropollutants (OMP) pose a significant environmental challenge, and microbial degradation research typically involves monitoring parent compound depletion and metabolite production. Previous studies on the antibiotic sulfamethoxazole (SMX) have shown its incomplete biotransformation by either mixed microbial communities or acclimated pure bacterial across various concentrations. However, the mechanisms behind this incomplete degradation and its relationship with the enzymatic capacities and expressions at environmentally relevant concentrations remain unclear. Therefore, this study investigated the biotransformation of SMX and the variations in the proteome at low µg L −1 concentrations using acclimated Microbacterium sp. BR1 as the bacterial degrader. Results show an incomplete depletion of the SMX and accumulation of the metabolite 3-amino-5-methylisoxazole (3A5MI). All test concentrations triggered the expression of the sulfonamide degrading enzymes (SadAB) and the modified target enzyme (Sul). Analysis of the functional proteins revealed increased cellular regulation and confirmed the bacterial strain's continued activity throughout the experiment. This suggests that at low SMX concentrations, even a highly sensitive and metabolically active strain may still require complementary enzymatic machinery to degrade potentially inhibitory metabolites. Thus, this study provides important insights into the persistence of SMX and reveals the complexities of antibiotic biodegradation at environmentally relevant concentrations, highlighting the need for comprehensive understanding of enzymatic mechanisms in micropollutant remediation strategies.