A 6-month-old infant presents with a 3-day history of fever, poor feeding, and irritability. On examination, the child is lethargic, has a bulging fontanelle, and neck stiffness. Lumbar puncture is performed. Cerebrospinal fluid (CSF) analysis shows: Appearance: Turbid, WBC: 1500 cells/mm³ (90% neutrophils), Protein: 2.5 g/L, Glucose: 1.0 mmol/L (Serum glucose: 5.0 mmol/L). What is the most likely causative organism?