In vitro antimicrobial inhibition patterns of nutritionally variant streptococci

Twenty-four isolates of nutritionally variant streptococci, previously categorized as species included among the viridans streptococci, were tested for susceptibility to 12 antimicrobial agents. Minimum inhibitory concentrations for these isolates and for a control group of viridans streptococci with no apparent nutritional deficiencies were determined in two microdilution systems. Pyridoxal hydrochloride, which enhances growth of the nutritional variants, was added to one of these microdilution systems but not to the other. An agar dilution method was also used to test the nutritionally variant isolates. Minimum inhibitory concentrations determined by the three methods compared favorably. Penicillin, nafcillin, methicillin, and clindamycin were less effective in vitro against the nutritional variants than were the other antibiotics tested. Streptomycin, however, was less effective against the control isolates.