Macrolide antibiotics differ significantly in their bactericidal activity. (A) Killing effect of fourfold MIC concentrations of ERY (magenta), TEL (dark blue), or SOL (light blue) against S. pneumoniae cells. The viable cells counts are indicated as colony forming units (CFU). (B) Survival of S. pneumoniae cells exposed for 18 h to various concentrations of ERY (magenta) or SOL (light blue). The experimental points determined at the fourfold MIC concentration of ERY or SOL are indicated by arrows. The dotted line marks the three-orders reduction in CFU, representing the operational definition of the bactericidal activity (1). (C) Survival of cells exposed for the indicated time periods to fourfold MIC of SOL (light blue), 40-fold MIC of Tet (orange), or fourfold MIC of SOL and 40-fold MIC of Tet (light blue/orange). In the latter case, cells were preincubated with Tet for 30 min before the addition of SOL. The error bars in A and C show SD in three independent experiments.