Antimicrobial peptides (AMPs) can target the bacterial envelope or alternatively have intracellular targets. The latter requires uptake of the peptide by the bacterial cells. The bacterial internalization of an AMP can be evaluated by a fluorescence-based method that couples the use of the fluorescently labelled AMP to the fluorescence quencher trypan blue. Trypan blue is excluded from the interior of intact cells and the fluorescence of the extracellular peptide or of the peptide bound on the bacterial surface can be quenched by it, while the fluorescence of the internalized peptide is not affected. The uptake of the peptide by the bacteria is determined by measuring the fluorescence in individual cells by flow cytometry.
