Reported herein is a relebactam-derived fluorogenic reagent for covalent labeling of serine β-lactamases (SBLs), which are the major causes of bacterial resistance to β-lactam antibiotics. This highly selective imaging reagent generates over 300-fold stronger near-infrared fluorescence signals upon covalently bonding to SBLs, allowing wash-free visualization of live antimicrobial-resistant bacteria.