The ineffectiveness of tobramycin combination therapy in Streptococcus faecium endocarditis

A patient required mitral valve replacement following ineffective antibiotic treatment of enterococcal endocarditis caused by Streptococcus faecium. Endocarditis had relapsed despite therapy with ampicillin and tobramycin for six weeks. A second relapse had occurred following treatment with penicillin and gentamicin. Initial failure of antibiotic therapy may be related to the known lack of in vitro and in vivo synergy between penicillin and tobramycin against S. faecium. Effective therapy of enterococcal endocarditis requires considerations of bacterial speciation, determination of high-level aminoglycoside resistance, and preferably adequate antibiotic synergy studies to assure effective therapy.