Viridans streptococcal endocarditis: the role of various species, including pyridoxal-dependent streptococci.

The viridans streptococci are a heterogeneous group of organisms frequently associated with microbial endocarditis. Isolates from consecutive patients with endocarditis due to viridans streptococci who were seen at The New York Hospital from 1944 to 1955 and from 1970 to 1978 were speciated, and the relative frequencies and patterns of sus ceptibility to penicillin were determined. Vitamin B6-dependent streptococci, classified as Streptococcus mitior, accounted for 5%-6% of microbial endocarditis in both time periods. Since these nutritional variants require the active forms of vitamin B6 for growth in routein media, they can be responsible for culture-negative endocarditis. Media must be supplemented with either pyridoxal hydrochloride or pyridoxamine dihydrochloride (the active forms of vitamin B6) for isolation, identification, and subsequent testing of susceptibility to antimirobial agents. Pyridoxine hydrochloride does not support the growth of these organisms in vitro.