MIC values do not predict the intraphagocytic killing of Staphylococcus aureus by naphthalenic ansamycins

Ten naphthalenic ansamycins were compared for their ability to kill extracellular or phagocytosed Staphylococcus aureus 502A. These included rifamycins, streptovaricins and tolypomycin Y. Although the compounds differed markedly in killing extracellular S. aureus, there was surprisingly little difference between them in assisting human leukocytes to kill phagocytosed S. aureus. In fact, when compared to rifampin, some ansamycins that were less effective in killing extracellular bacteria were more effective in killing phagocytosed bacteria. These data, together with an analysis of structure and activity, suggested that a specific transport mechanism might be involved. First considered was a vitamin K transport mechanism. Indeed warfarin, a vitamin K antagonist, blocked the ability of rifampin to kill phagocytosed S. aureus, as did the coumarins, novobiocin and coumarin-3-carboxylic acid. However, direct evidence for a vitamin K transport mechanism could not be obtained using vitamin K preparations. The fused phenolic, bicyclic system common to all of these ansamycins was tentatively considered to be the portion necessary for phagocyte penetration.