A 2 years old child is brought to the emergency with history of fever and vomiting. On examination he has neck rigidity. CSF examination shows polymorphs more than 200 /ml; protein 100 mg/dl and glucose 10 mg/dl. the Gram's stain shows the presence of Gram-negative coccobacilli. The culture shows of bacteria only on chocolate. Agar and not on blood agar. The causative agent is: