An 83-year-old man with Parkinson's disease presents with low-grade fever and cough for several weeks. Lately, he has been experiencing more rigidity and difficulty with his walking. He is on a levodopa/carbidopa combination for treatment for the past 5 years. On examination, his gait is shuffling and slow. He has a tremor in his left hand at rest, and there is cogwheel rigidity of the forearm. There are crackles in the left lower lung field. CXR reveals a lung abscess in the left lower lobe. Which of the following is the most likely bacteriologic diagnosis for the lung abscess?