A 2 years old child is brought to the emergency with history of fever and vomiting. On examination he has neck rigidity. The CSF examination shows polymorphs more that 2000/ml protein 100 mg/dl and glucose 10mg/dl. The Gram stain shows the presence of Gram negative coccobacilli. The culture shows growth of bacteria only on chocolate agar and not on blood agar. The causative agent is -