Infective endocarditis produced by organisms that do not quickly or readily grow in blood cultures, usually because their growth is masked by the previous use of antibiotics or because the causative organisms require special culture media or grow slowly in the laboratory. Mycoplasma, Ricksettsia, HACEK (an acronym for Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, Kingella) organisms, and some fungi produce culture-negative endocarditis.