A 72-year-old woman with diabetes mellitus type 2 is admitted to the ICU on day 5 of mechanical ventilation following elective abdominal surgery. She develops fever (39.2°C), purulent endotracheal secretions, and a new infiltrate on chest X-ray in the right lower lobe. Blood cultures are negative. Sputum culture grows Acinetobacter baumannii resistant to all beta-lactams and fluoroquinolones. What is the most appropriate antimicrobial therapy?