Antimicrobial Susceptibilities of Species of Viridans Streptococci

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Abstract
Sixty-three strains of viridans streptococci (12 strains of Streptococcus mutans, 12 of Streptococcus mitis, nine of Streptococcus sanguis type I, 12 of S. sanguis type II, 10 of Streptococcus MG-intermedius, and eight of Streptococcus anginosus-constellatus) were tested for susceptibility to 12 antibiotics by a microdilution method. The antibiotics tested were: penicillin, ampicillin, nafcillin, cephalothin, cefamandole, vancomycin, streptomycin, kanamycin, amikacin, netilmicin, gentamicin, and rifampin. The minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) extended over a 512-fold range. Penicillin G was the most active antimicrobial agent, with MICs of 0.06–4 μg/ml and MBCs of 0.06–>8 μg/ml. A concentration of 0.06 μg/ml was inhibitory and bactericidal, respectively, to 12 and 12 of the 12 strains of S. mutans, five and four of the 12 strains of S. mitis, nine and four of the nine strains of S. sanguis type I, nine and four of the 12 strains of S. sanguis type II, nine and four of the 10 strains of Streptococcus MG-intermedius, and six and two of the eight strains of S. anginosus-constellatus. The activity of ampicillin was similar to that of penicillin G, but cephalothin and cefamandole were less active; 0.125 and 0.5 μg/ml, respectively, inhibited and killed 50% of the 63 strains. Vancomycin (1 μg/ml) inhibited 54 and killed 19 strains. Gentamicin and netilmicin were the most active of the five aminoglycosides tested, but at a concentration of 8 μg/ml, they inhibited and killed, respectively, only 37% and 15% of the 63 strains. Rifampin (0.06 μg/ml) inhibited 49 but killed only seven strains. In general, S. mitis and S. sanguis type II required higher concentrations of antibiotics for inhibition and killing than did the other species of viridans streptococci tested.