Failure of medical therapy in tricuspid endocarditis due to Staphylococcus aureus

A patient with isolated tricuspid endocarditis due to Staphylococcus aureus had persistent bacteremia despite antibiotic therapy for 47 days until surgery. Cultures of the resected valve disclosed 1.4 X 10(8) colony forming units per gram of valve. Time-kill studies done with the strain of S. aureus and the antibiotics used in the therapy of this patient disclosed decreased bactericidal activity of these antibiotics with increasing inocula concentrations. At 10(8) inoculum, only the combination of vancomycin and tobramycin was bactericidal. The inoculum dependence of bactericidal activity might explain the lack of correlation between in vitro susceptibility tests with clinical outcome of some endocarditis patients.