A 7-year-old girl with a history of frequently relapsing nephrotic syndrome presents to the emergency department with a fever of 39.1°C (102.4°F), diffuse abdominal pain, and rebound tenderness. A paracentesis is performed, and the fluid analysis shows a leukocyte count of 400 cells/μL. This presentation is highly suggestive of spontaneous bacterial peritonitis. Which organism is most commonly responsible for this complication?