A 59-year-old man undergoes coronary bypass surgery. He receives vancomycin prophylactically for 24 hours. On the ninth postoperative day, he develops a fever of 39.8degC (103degF) with a heart rate of 115 beats/min and a blood pressure of 105/65 mm Hg. The surgical site is healing well with no redness or discharge. His white blood cell count is 14,000/mm3 and urinalysis reveals many white blood cells per high-power field. Blood and urine cultures grow a non-lactose-fermenting oxidase-positive gram-negative rod. Which of the following antibiotics is most appropriate to treat this infection?