A patient who had surgery to put in a pacemaker and who states he felt fine for the first 2 months now presents 3 months postoperatively with complaints of malaise and increasing fatigue. He is running a low-grade fever, tires easily, and has worsening heart murmurs. Which of the following staphylococcal organisms causes subacute bacterial endocarditis that generally occurs 2 months or more after heart surgery?