A Rapid Fluorescence-Based Microplate Assay to Investigate the Interaction of Membrane Active Antimicrobial Peptides with Whole Gram-Positive Bacteria.

BACKGROUND

Membrane-active antimicrobial peptides (AMPs) are interesting candidates for the development of novel antimicrobials. Although their effects were extensively investigated in model membrane systems, interactions of AMPs with living microbial membranes are less known due to their complexity. The aim of the present study was to develop a rapid fluorescence-based microplate assay to analyze the membrane effects of AMPs in whole and .

METHODS

Bacteria were exposed to bactericidal and sub-inhibitory concentrations of two membrane-active AMPs in the presence of the potential-sensitive dye 3,3'-dipropylthiadicarbocyanine iodide (diSC(5)) and the DNA staining dye propidium iodide (PI), to simultaneously monitor and possibly distinguish membrane depolarization and membrane permeabilization.

RESULTS

The ion channel-forming gramicidin D induced a rapid increase of diSC(5), but not PI fluorescence, with slower kinetics at descending peptide concentrations, confirming killing due to membrane depolarization. The pore-forming melittin, at sub-MIC and bactericidal concentrations, caused, respectively, an increase of PI fluorescence in one or both dyes simultaneously, suggesting membrane permeabilization as a key event.

CONCLUSIONS

This assay allowed the distinction between specific membrane effects, and it could be applied in the mode of action studies as well as in the screening of novel membrane-active AMPs.