A 65-year-old man with a long history of diabetes mellitus was hospitalized for treatment of an ulcer, which had been present on his left great toe for several months. Left sided below knee amputation was performed because of the inability multiple courses of oral antibiotics to resolve the ulcer, On the first postoperative day, he developed fever which worsened by day 2. His amputation stump was mottled with many areas of purplish discoloration, and the most distal areas were quite obviously necrotic. Crepitus was palpable up to his patella. An X-ray of the left lower extremity was done. What is the likely pathogen?