A 64-year-old man, who is a chronic alcoholic, presents with fever, chills and increasing shoness of breath. The patient appears in acute respiratory distress and complains of pleuritic chest pain. Physical examination shows crackles and decreased breath sounds over both lung fields. The patient exhibits tachypnea, with flaring of the nares. The sputum is thick, mucoidy and blood-tinged. Which of the following pathogens is the most common cause of this patient's pulmonary infection?