A 28-year-old woman is admitted with 60% TBSA deep partial and full-thickness burns sustained 8 hours ago. She has been resuscitated with Parkland formula fluids. On day 3, she develops sudden-onset fever (39.2°C), tachycardia, and a change in burn wound appearance with rapid eschar separation. Which investigation is most appropriate to identify the causative organism and guide antimicrobial therapy?