A 32-year-old man presents with acute onset confusion, headache, and fever. He has a history of intravenous drug use. On examination, he is febrile (39.0°C), tachycardic (120 bpm), and has a new pansystolic murmur loudest at the left lower sternal edge. CT head is normal. Lumbar puncture shows: WCC 200/mm³ (90% polymorphs), Protein 1.2 g/L, Glucose 2.5 mmol/L (plasma glucose 6.0 mmol/L). What is the most likely causative organism?