A 58-year-old man undergoes elective open cholecystectomy for chronic cholecystitis. The procedure is uncomplicated and lasts 45 minutes. On postoperative day 4, he develops fever (38.5°C), erythema, warmth, and purulent drainage from the surgical wound. Gram stain of the drainage shows gram-positive cocci in clusters. Which of the following is the MOST appropriate immediate management?