A 52-year-old woman has had a chronic cough for the past 2 years, accompanied by a small amount of occasionally blood-streaked, whitish sputum. On physical examination, her temperature is 37.9deg C, pulse is 72/min, respirations are 22/min, and blood pressure is 125/80 mm Hg. Crackles are heard on auscultation over the upper lung fields. Heart sounds are faint, and there is a 15 mm Hg inspiratory decline in systolic arterial pressure. The chest radiograph shows prominent heart borders with a "water bottle" configuration. Pericardiocentesis yields 200 mL of the bloody fluid. Infection with which of the following organisms is most likely to produce these findings?