A 7-year-old boy presents with a 2-day history of fever, headache, vomiting, and reluctance to bend his neck forward. Examination reveals neck stiffness and a positive Kernig's sign. He has received all his routine childhood immunisations. Lumbar puncture is performed. CSF analysis shows: Appearance - Turbid, White Cell Count - 1500 cells/mm³ (90% neutrophils), Protein - 1.8 g/L (High), Glucose - 1.5 mmol/L (Low, corresponding plasma glucose 6.0 mmol/L). What is the most likely causative organism group?