A mother brings her 1-month-old infant to the pediatrician. She says the baby is crying more than usual and is vomiting and does not want to eat. Meningitis is suspected, and a lumbar puncture is done, which shows the following results; Opening pressure = 240 mm H2O (normal = 100-200 mm H2O), WBC count: 1200/mm3, Protein: 200 mg/dL, Glucose: 30 mg/dL, Gram stain: gram-positive rods, W hich of the following organisms is most likely responsible for this infant's meningitis?