Antimicrobial therapy of vitamin B6-dependent streptococcal endocarditis.

Vitamin B6-dependent viridans streptococci were isolated from two 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 as well as in experimental endocarditis. These laboratory findings confirmed the clinical observations in these two patients that penicillin-streptomycin therapy should be used in vitamin B6-dependent streptococcal endocarditis. Nutritionally varient streptococci may be important pathogens in microbial endocarditis and must be considered in patients with suggestive clinical findings but negative blood cultures.