A 42-year-old woman from Delhi presents with a 3-day history of fever and productive cough. She has chronic obstructive pulmonary disease (COPD) and was hospitalized 2 weeks ago for pneumonia, treated with ceftriaxone for 7 days. Chest X-ray shows right lower lobe consolidation. Sputum culture grows a gram-negative rod. The organism is resistant to ceftriaxone, cefotaxime, ceftazidime, and piperacillin-tazobactam. It is susceptible to carbapenems and colistin. Clavulanic acid does not inhibit the organism's beta-lactamase. What is the most likely resistance mechanism?