The causative microorganisms vary, in part because of different portals of entry. In native valve endocarditis (NVE), viridans streptococci, staphylococci, and HACEK organisms (Haemophilus spp., Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, Kingella kingae) enter the bloodstream from oral, skin, and upper respiratory tract portals, respectively. Streptococcus bovis originates from the gut and is associated with colon polyps or cancer. Enterococci originate from the genitourinary tract. Nosocomial endocarditis, frequently due to Staphylococcus aureus, arises most often from bacteremia related to intravascular devices. Prosthetic valve endocarditis (PVE) developing within 2 months of surgery is due to intraoperative contamination or a bacteremic postoperative complication and is typically caused by coagulase-negative staphylococci (CoNS), S. aureus, facultative gram-negative bacilli, diphtheroids, or fungi. At 1 year after valve surgery, endocarditis is caused by the same organisms that cause community-acquired NVE. IV drug users are particularly prone to tricuspid valve endocarditis caused by S. aureus (often a methicillin-resistant strain); they are also at risk for left-sided endocarditis caused by S. aureus, Pseudomonas aeruginosa, or Candida spp. Fastidious organisms such as the nutritionally variant bacteria Granulicatella and Abiotrophia, HACEK bacteria, Bartonella spp., Coxiella burnetii, Brucella spp., and Tropheryma whipplei can cause culture-negative endocarditis. β-Hemolytic streptococci, S. aureus, and pneumococci typically cause acute endocarditis, while viridans streptococci, enterococci, CoNS, and HACEK organisms usually cause subacute disease.