Optimal microbial inoculant dosage decouples antibiotic removal from resistance gene attenuation during sludge composting.

The application of microbial inoculants during composting represents a promising strategy to mitigate antibiotic residues and antibiotic resistance genes (ARGs) in organic wastes. However, the dose-dependent effects of inoculation on antibiotic degradation and ARG fate remain poorly understood. This study investigated the impacts of low (0.03%, w/w; T2) and high (0.15%; T3) dosages of a microbial consortium on the fate of three antibiotic (oxytetracycline [OTC], ciprofloxacin [CIP], and sulfamerazine [SM1]) and their corresponding ARGs during 43-day sludge composting. Results showed that T2 achieved superior removal of all three antibiotics (78.62% for OTC, 78.73% for SM1, 42.83% for CIP) compared to T3 (72.31%, 73.24%, and 31.14%, respectively). However, ARG dynamics exhibited gene-specific responses: T2 unexpectedly caused poor sul2 removal (28.13%) and substantial tetX enrichment (657.61%), whereas T3 enhanced the removal of tetA (75.12%), tetG (59.06%), and intI1 (96.09%). In CK, T1, and T3, sul2 removal was efficient (91.2-95.9%). Mechanistic analyses revealed that T3's brief but intense thermophilic peak (65.2 °C for 8  days) potentially inactivated mesophilic degrading bacteria and reduced antibiotic bioavailability, while its stable enrichment of Bacillus and Actinomadura facilitated ARG host suppression. Degradation pathway analysis identified multiple intermediates for each antibiotic, confirming complex, multi-step transformation processes. Redundancy analysis and structural equation modeling identified temperature, Actinobacteria abundance, and MGEs as key drivers of ARG dynamics. These findings demonstrate that antibiotic removal and ARG attenuation are not necessarily coupled during composting, highlighting the critical need for optimizing inoculant dosage to simultaneously achieve efficient pollutant degradation and effective resistance control.