XF-73, a novel antistaphylococcal membrane-active agent with rapid bactericidal activity

Objectives: XF-73 is a novel porphyrin antibacterial agent previously reported to inhibit a range of Gram-positive bacterial species, including Staphylococcus aureus. Its mode of action is unknown. Using S. aureus as a model organism we sought to examine the basis of its antibacterial activity. Methods: The effects of XF-73 on the growth and survival of S. aureus SH1000 were investigated by viable count and culture absorbance techniques. Inhibition of macromolecular synthesis and disruption of membrane integrity after exposure to XF-73 were examined by radiolabelling experiments, the BacLight fluorescent dye assay and measurement of K+ and ATP leakage from the cell. The effect of XF-73 on a staphylococcal coupled transcription -translation system was also investigated. Results: XF-73 was rapidly bactericidal against S. aureus SH1000 and demonstrated more rapid killing kinetics than all other comparator agents when tested at an equivalent multiple (4x) of the MIC. Exposure of S. aureus to XF-73 for 10 min completely inhibited DNA, RNA and protein synthesis. XF-73 had no effect on transcription and translation in vitro. Cells exposed to XF-73 gave a positive response in the BacLight assay, which detects membrane damage. The drug also caused substantial loss of K+ and ATP from the cell, but did not promote bacterial lysis. Conclusions: XF-73 exhibited rapid membrane-perturbing activity, which is likely to be responsible for inhibition of macromolecular synthesis and the death of staphylococci exposed to the drug.