Clinical Case The subject was a 30-year-old male addict with a history of intravenous substance abuse. In 2002, he had infectious endocarditis caused by methicillin-resistant Staphylococcus aureus, which displayed a good response to antibiotic treatment. In April 2003, he suffered a motor vehicle accident that produced a femoral fracture requiring the insertion of an intramedullary spike. A week before hospitalization, he showed symptoms of general weakness, fever, and chills, and during an examination, a 2 to 3/6 systolic murmur was detected. Because of the physical results and his previous history, which included tricuspid valve endocarditis, infectious endocarditis was suspected. Blood was drawn for culture and treatment was started empirically with 500 mg of vancomycin plus 400 mg of ciprofloxacin i.v. twice a day. A transthoracic echocardiogram showed normal movement of the intraventricular septum but a lack of movement in the medial and apical septa, with a slight deterioration of systolic function and no vegetative growths. It also showed an irregular surface on the tricuspid valve, a serious level of insufficiency, and dilated right-side cavities. Initial laboratory results were as follows: erythrocyte sedimentation rate, 52 mm/h, and white blood cell count, 9,400/mm. The HIV serology was negative. Despite antibiotic treatment, after 1 week of hospitalization, the patient was still febrile with chills. A week from the date of hospitalization, a transesophageal echocardiogram showed a rounded and irregularly edged vegetative growth with a small pedicle on the left auricle, over the interauricular septum. Diagnosis was a suspected rupture of the interauricular septum and moderate tricuspid failure. Because of the persistence of fever and the fact that the patient had a compromised immune system and a history of hospitalization, ciprofloxacin was replaced with 500 mg of imipenem every 6 h in addition to the twice-daily dose of 500 mg of vancomycin. After a clear and rapid clinical recovery, the patient again became febrile. In four blood culture samples (Laboratorios Britania, Argentina), processed by a standard methodology, gram-negative diplococci were observed, which after 24 h of cultivation on blood agar, developed into small, regular, convex, and