A 50-year-old man with a neurodegenerative disease has had a fever and cough productive of yellow sputum for the past 3 days. On physical examination, there is dullness to percussion at the left lung base. A chest radiograph shows areas of consolidation in the left lower lobe. Despite antibiotic therapy, the course of the disease is complicated by abscess formation, and he dies. At autopsy, there is a bronchopleural fistula surrounded by a pronounced fibroblastic reaction. Small, yellow, 1-cm to 2-mm "sulfur granules" are grossly visible within the area of abscess formation. Which of the following organisms is most likely to produce these findings?