After infecting mice with an experimental strain (E. coli strain into which a plasmid encoding class B β-lactamase (IMP-1) is introduced), Metallo-beta-lactamase ligand 1 (compound A; 100 mg/kg; intraperitoneal injection) is administered, and life and death are observed. Administration of Metallo-beta-lactamase ligand 1 (compound A) significantly improves the survival rate.