A 62-year-old woman undergoes emergency exploratory laparotomy for perforated peptic ulcer. The procedure takes 2 hours and involves significant contamination with gastric contents. She receives intraoperative cefazolin 2 g IV. On postoperative day 6, she develops fever (39.2°C), tachycardia, and abdominal tenderness with guarding. Imaging reveals a 4 cm fluid collection in the pelvis. Which organism is MOST likely responsible for this deep surgical site infection (organ-space SSI)?