Bactericidal activity of combinations of penicillin or clindamycin with gentamicin or streptomycin against species of viridans streptococci.

Checkerboard bactericidal studies were performed with two strains each of Streptococcus mitis, S. salivarius, and S. sanguis, and the results were analyzed by the use of isobolograms. Synergy between penicillin and streptomycin or gentamicin was demonstrated with four strains, representing two of S. salivarius, one of S. sanguis, and one of S. mitis; the remainder were either too susceptible or too resistant to be analyzed. Clindamycin with streptomycin or gentamicin acted synergistically with three strains, representing two of S. sanguis and one of S. salivarius; the remainder were either too susceptible or too resistant to be analyzed.