A 6-month-old infant presents with a 3-day history of fever, poor feeding, and irritability. On examination, the infant appears lethargic, has a bulging fontanelle, and neck stiffness is suspected. Lumbar puncture is performed. CSF analysis shows: Appearance: Turbid, White cell count: 1500 cells/mm³ (90% neutrophils), Protein: 2.5 g/L (High), Glucose: 1.0 mmol/L (Low, corresponding blood glucose 5.0 mmol/L). What is the most likely diagnosis?