Penicillin tolerance in nutritionally variant streptococci.

Eleven strains of nutritionally variant streptococci were tested for their susceptibility to penicillin by a broth dilution method. All minimal inhibitory concentrations were low. It was found that plates containing only vitamin B6 (pyridoxal HCl) and cysteine did not reveal true minimal bactericidal concentrations. When penicillinase was added to the subculture medium and a staphylococcal streak was made across the plates, all 11 strains were found to have high minimal bactericidal concentrations and were shown to be tolerant to penicillin.