A 13-day-old infant is brought to the physician with a 4-day history of fever and irritability. The mother reports that the infant was born full-term via normal vaginal delivery. Physical examination reveals a temperature of 38.2°C, a heart rate of 160/min, and a respiratory rate of 50/min. There is no rash or other skin lesions. Cerebrospinal fluid (CSF) studies show a white blood cell count of 185 cells/µL (0-22), a protein level of 45 mg/dL (19-140), a glucose level of 50 mg/dL (17-162). Empirical antibiotics were started. Later culture results were positive for group B streptococcus and a repeat lumbar puncture showed CSF sterilization. Which of the following is the most appropriate next step in management?