A mother brings her 1-year-old daughter to the physician. She says that for the last 2 days her daughter has been fussy and crying more than usual. She also refuses formula. The patient has a fever of 39.4degC (102.9degF). Meningitis is suspected, and a lumbar puncture is per- formed. Analysis of the cerebrospinal fluid shows an opening pressure of 98 mm H2O, a leukocyte count of 1256/mm3, a protein level of 210 mg/dL, and a glucose level of 31 mg/dL.. The mother says that the patient has received no immunizations. Which of the following organisms is most likely responsible for this patient's illness?