Effect of mixing on rifampin bactericidal activity against staphylococci

Minimal bactericidal concentrations of rifampin were significantly increased, and serum bactericidal activity from volunteers receiving this drug was significantly decreased by vigorous mixing of microtiter plates before sampling when tested against Staphylococcus aureus and Staphylococcus epidermidis at 10(5) and 10(6) colony-forming units per ml. These results suggest that microtiter estimates of the bactericidal activity of rifampin against staphylococci should be performed after vigorous shaking.