A 62-year-old man with diabetes mellitus presents to the emergency department with fever (38.9°C), dysuria, and flank pain of 3 days' duration. Urinalysis shows pyuria and bacteriuria. Urine culture grows a gram-negative rod identified as Escherichia coli. Antibiotic susceptibility testing reveals resistance to ceftriaxone and cefotaxime, but susceptibility to imipenem and meropenem. The organism is ESBL-positive on phenotypic confirmatory testing. The patient is currently on intravenous fluids and empiric cefotaxime. What is the most appropriate immediate next step in management?