Phosphate Ions Alter the Binding of Daptomycin to Living Cell Membranes

Advancements in antibiotic drug design are often hindered by missing information in how these small molecules interact with living cells. The antibiotic, daptomycin, has found clinical success, and emerging resistance, but a comprehensive picture of its mechanism of action has remained elusive. Using the surface-specific spectroscopy, second harmonic generation, we are able to quantitatively assess the binding of daptomycin to living cell membranes without the addition of exogenous labels. Our results reveal similar binding affinities for both gram-positive and gram-negative bacteria studied, including E. coli. More importantly, we show that phosphate ions influence the binding of daptomycin to the gram-positive bacterium E. faecalis. The role of environmental phosphate has not previously been considered in any proposed mechanism and its implications are expected to be important in vivo.