A 28-year-old man with a history of intravenous drug use presents with fever (39.2°C), a new systolic murmur, and multiple splinter hemorrhages on his fingernails. Blood cultures are pending. While awaiting culture results, the microbiology team performs a Gram stain on the blood culture bottle, which shows gram-positive cocci in clusters. Which investigation is most appropriate to rapidly confirm *Staphylococcus aureus* bacteremia and differentiate it from *Staphylococcus epidermidis*?