Antimicrobial Therapy of Vitamin B_6-Dependent Streptococcal Endocarditis

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Abstract
Vitamin B6-dependent viridans streptococci were isolated from 2 patients with microbial endocarditis. Because of their unique requirement for pyridoxal hydrochloride, these organisms did not grow normally in the media usually used in diagnostic laboratories. When tested in supplemented media, both strains were resistant to penicillin G and relatively sensitive to streptomycin. Penicillin-streptomycin synergy was demonstrated in vitro and in experimental endocarditis. These laboratory findings confirmed the clinical observation in these 2 patients that penicillin-streptomycin therapy should be used in vitamin B6-dependent streptococcal endocarditis. Nutritionally variant streptococci may be important pathogens in microbial endocarditis and must be considered in patients with suggestive clinical findings but negative blood cultures.