Wash-free, peptide-based fluorogenic probes for microbial imaging

Peptides with pan-antimicrobial affinity were synthesized and decorated with environmentally sensitive fluorophores nitrobenzoxadiazole (green) and merocyanine (red). The labeling efficacies against a range of clinically relevant fungal and Gram-negative and positive bacterial species (Candida albicans, Escherichia coli, and Staphylococcus aureus) were explored. The fluorogenic probes containing exclusively l or d-amino acids showed rapid and efficient labeling of all microbial species, whereas probes with a mixture of l or d-amino acids failed to label fungi or bacteria, highlighting the importance of the alpha-helical peptide structure for interaction with the microbial cell membrane. Importantly, the nature of the dye allowed fluorescence detection/labeling without the need for a wash step, paving the way for direct application of the probes.