This invention provides novel β-lactamase inhibitors of the aryl- and heteroarylsulfonamidomethylphosphonate monoester class having nitrogen-based cations or quarternary ammonium groups. The compounds inhibit three classes of β-lactamases and synergize the antibacterial effects of β-lactam antibiotics (e.g., imipenem and ceftazidime) against those micro-organisms normally resistant to the β-lactam antibiotics as a result of the presence of the β-lactamases. Formula (I) or pharmaceutically acceptable salt thereof.