Vitamin B6-dependent Streptococcus mitior (mitis) isolated from patients with systemic infections

Strains of nutritional-variant viridans Streptococcus were isolated from two patients with bacterial endocarditis and from one patient with a pancreatic abscess. All three strains grew as satellite colonies of other bacteria but did not grow as pure cultures in media lacking sufficient thiol compounds. For one-half maximal growth these organisms required the addition of one of the active forms of the B6 vitamin, pyridoxal with HCl (0.16-1.45 mug/ml) or pyridoxamine dihydrochloride (0.67-2.0 mug/ml), or addition of l-cysteine (0.235-0.425 mg/ml) to routine bacteriological media. The active forms of vitamin B6 are essential coenzymes in the synthesis of l-cysteine and of other thiol compounds. Use of media supplemented with 0.001 percent pyridoxal with HCL led to identification of the strains as Streptococcus mitior (mitis). Incorporation of pyridoxal with HCl, pyridoxamine dihydrochloride, or l-cysteine into media is recommended for the isolation and identification of vitamin B6-dependent viridans streptococci.