The polyspecificity and adaptability of multidrug resistance shaped the antibiotic resistome in anaerobic digestion amended by nZVI: Evidence from efflux pumps and rRNA methyltransferase genes

•nZVI exposure increased the antibiotic resistome in anaerobic digestion.•Efflux pump and rRNA methyltransferase shaped the antibiotic resistome variation.•Improved adaptability of efflux pump benefited the anaerobes withstanding nZVI.•Increased acquisition of rRNA methyltransferase preserved the structure of protein.•Polyspecificity of multidrug resistance drove the ARGs evolution in anaerobes.